2 1

1. It comes from that fact that the difference operator Δf(n) = f(n + 1) − f(n) acting on polynomials f(x) of degree d will result in polynomials in degree d − 1 (check this!) - the difference between f(n) = 12 + 22 + ⋯ + n2 and f(n + 1) = 12 + ⋯ + (n + 1)2 is simply (n + 1)2, which is a quadratic in n, hence we should expect f to be ...

2 1. Fraction calculator. 1/2 x 1/2. This fraction calculator performs basic and advanced fraction operations, expressions with fractions combined with integers, decimals, and mixed numbers. It also shows detailed step-by-step information about the fraction calculation procedure. The calculator helps in finding value from multiple fractions operations.

Rules for expressions with fractions: Fractions - use a forward slash to divide the numerator by the denominator, i.e., for five-hundredths, enter 5/100.If you use mixed numbers, leave a space between the whole and fraction parts. Mixed numerals (mixed numbers or fractions) keep one space between the integer and fraction and use a forward slash to input …

Free math problem solver answers your algebra homework questions with step-by-step explanations.2.1 Scores. 2.1.1 Complete; 2.1.2 Selections; 2.2 Arrangements and Transcriptions. 2.2.1 Complete. 2.2.1.1 For Orchestra (Brandes) 2.2.1.2 For Strings (Choe) 2.2.1.3 For 2 Violins, Viola and Cello (Brand) 2.2.1.4 For 2 Pianos (Besseling) 2.2.1.5 For 2 Pianos (Bouillet) 2.2.1.6 For Piano 4 hands (Köhler) 2.2.1.7 For Piano 4 Hands (Markull) 2.2. ...Fraction calculator. 1/2 x 1/2. This fraction calculator performs basic and advanced fraction operations, expressions with fractions combined with integers, decimals, and mixed numbers. It also shows detailed step-by-step information about the fraction calculation procedure. The calculator helps in finding value from multiple fractions operations.2>&1 is an I/O redirection operator used in Bash that redirects the stderr stream to the same destination as the stdout stream. In other words, it merges the error …Alpha builds are unstable and subject to change, but may offer the latest versionThe geometric series on the real line. In mathematics, the infinite series 1 2 + 1 4 + 1 8 + 1 16 + ··· is an elementary example of a geometric series that converges absolutely. The sum of the series is 1. In summation notation, this may be expressed as. The series is related to philosophical questions considered in antiquity, particularly ...Explanation: For example, if you split 12 as a 2:1 ratio, you get 8 and 4 (one part twice is large as the other). In general terms, you can work out the values like this: Ratio:a:b. N umber = x. = ( x a +b).a:( x a + b).b. Answer link. A ratio is a comparison between two (or more) different quantities of the same unit.2023-12-04. Update 2.1 for Cyberpunk 2077 and Phantom Liberty on PC, PlayStation 5 and Xbox Series X|S is coming out tomorrow (December 5th) around 12 PM CET. The update brings our usual bug fixes, changes and gameplay improvements. It also adds some of the most-requested features, such as the NCART metro, radioport, repeatable car races ...

Evaluate 1/2-1. Step 1. To write as a fraction with a common denominator, multiply by . ... Step 4.2. Subtract from . Step 5. Move the negative in front of the fraction. Algebra. Simplify 2 1/2. 2 1 2 2 1 2. A mixed number is an addition of its whole and fractional parts. 2+ 1 2 2 + 1 2. Add 2 2 and 1 2 1 2. Tap for more steps... 5 2 5 2. The result can be shown in multiple forms. Fraction calculator. 2 x 1/2. This fraction calculator performs basic and advanced fraction operations, expressions with fractions combined with integers, decimals, and mixed numbers. It also shows detailed step-by-step information about the fraction calculation procedure. The calculator helps in finding value from multiple fractions operations. HDMI ® 2.1b Specification. HDMI ® Specification HDMI 2.1b is the most recent update of the HDMI ® specification and supports a range of higher video resolutions and refresh rates including 8K60 and 4K120, and resolutions up to 10K. Dynamic HDR formats are also supported, and bandwidth capability is increased up to 48Gbps. Supporting the 48Gbps …This is how to do the equation: 6 / 2 (2 + 1) = x to get the right answer of 9. This is the correct way. I also show how people are getting the wrong answer ...Titus 2:1-8. New International Version. Doing Good for the Sake of the Gospel. 2 You, however, must teach what is appropriate to sound doctrine. 2 Teach the older men to be temperate, worthy of respect, self-controlled, and sound in faith, in love and in endurance. 3 Likewise, teach the older women to be reverent in the way they live, not to be ...

20 hours ago · 2-1-1 Information & Referral Search. 2-1-1 provides free and confidential information and referral. Call 2-1-1 for help with food, housing, employment, health care, counseling and more. Learn more about your local 2-1-1 by looking it up here. Free math problem solver answers your algebra homework questions with step-by-step explanations.Sum of the series 2^0 + 2^1 + 2^2 +…..+ 2^n. Given an integer N, the task is to find the sum of series 2 0 + 2 1 + 2 2 + 2 3 + …. + 2 n. A naive approach is to calculate the sum is to add every power of 2 from 0 to n. An efficient approach is to find the 2^ (n+1) and subtract 1 from it since we know that 2^n can be written as:System. After the Genshin Impact Version 2.1 update, each top-up level of Genesis Crystals in the Crystal Top-Up page will be reset to provide a double bonus for the first top-up. After the reset, each top-up level will be reset to provide a double bonus upon your first top-up. Top-up bonuses that have not been used before the reset will not be ...

Secret double octopus.

Rules for expressions with fractions: Fractions - use a forward slash to divide the numerator by the denominator, i.e., for five-hundredths, enter 5/100.If you use mixed numbers, leave a space between the whole and fraction parts. Mixed numerals (mixed numbers or fractions) keep one space between the integer and fraction and use a forward slash to input … The upper division is commonly abbreviated to "2:1" or "II.i" (pronounced two-one). The 2:1 is a minimum requirement for entry to many postgraduate courses in the UK. It is also required for the award of a research council postgraduate studentship in the UK, although a combination of qualifications and experience equal to a 2:1 is also acceptable. 6/2(2+1) Natural Language; Math Input; Extended Keyboard Examples Upload Random. Compute answers using Wolfram's breakthrough technology & knowledgebase, relied on by millions of students & professionals. For math, science, nutrition, history, geography, engineering, mathematics, linguistics, sports, finance, music…The Aether II for Minecraft 1.12.2 is not considered stable yet, but is playable. We recommend staying away from adding the Aether to modpacks or making showcase videos quite yet, as you may run into serious bugs or progression issues. Summarize IPs. Use our data visualization tool to create a visual overview of multiple IPs. Full IP address details for 2.2.2.1 (AS3215 Orange S.A.) including geolocation and map, hostname, and API details. The 2-1-1 Arizona Information and Referral Services program was founded in 1964 as Community Information and Referral Services and incorporated as a private, nonprofit 501 (c) (3) organization in 1979. In 2017, Solari, Inc. acquired the program. 2-1-1 Arizona Information and Referral Service operates 24 hours per day, seven days per week and ...

Apr 10, 2024 · The Yoga 9i Gen 8 is a premium swivel-hinge 2-in-1 perfect for anyone who needs touch-input, handwriting, and drawing features but doesn't want to compromise on the laptop experience. The dazzling ... Sum of the series 2^0 + 2^1 + 2^2 +…..+ 2^n. Given an integer N, the task is to find the sum of series 2 0 + 2 1 + 2 2 + 2 3 + …. + 2 n. A naive approach is to calculate the sum is to add every power of 2 from 0 to n. An efficient approach is to find the 2^ (n+1) and subtract 1 from it since we know that 2^n can be written as:2'1 is 63.5 cm or 0.635 m. One inch is 2.54 centimeters so CM = 2.54 * (2 * 12 + 1). Some countries use feet and inches to measure height and others use centimeters or meters. This calculates twofootone to cm. What percentile height is a …9 ou 1 ? On fait le point sur ce calcul devenu viral.Illustrations : Chloé Bouchaour https://www.instagram.com/chloescope_/Sources :Cajori Florian - …Quick look. $209.99$20999. HP 2023 Premium 2-in-1 Convertible Laptop, 11" HD IPS Touchscreen, Intel 4-Core Pentium Processor Up to 3.30GHz, 8GB Ram, 128GB SSD, Super-Fast 6th Gen WiFi, HDMI, Windows 11 (Renewed) 4.0 out of 5 s. Shop a wide selection of 2 in 1 Laptops at Amazon.com. Free shipping and free returns on eligible …Met de linker Alt toets en numeriek toetsenbord [?] AltGr 5. Met de Alt Gr-toets. Mac. Geen sneltoets hiervoor. Selecteer de ½ uit het `Speciale tekens` venster. (alt⌥ + cmd⌘ + T) …The 2-1-1 Arizona Information and Referral Services program was founded in 1964 as Community Information and Referral Services and incorporated as a private, nonprofit 501 (c) (3) organization in 1979. In 2017, Solari, Inc. acquired the program. 2-1-1 Arizona Information and Referral Service operates 24 hours per day, seven days per week and ...To ensure coverage continues, if you're eligible, report changes (contact information, pregnancy, etc.) as soon as possible. Log into your account at yourtexasbenefits.com or call 2-1-1 and select, Option 2. End of Continuous Medicaid Coverage | Texas Health and Human Services. For information about COVID-19, call 2-1-1, Option 1. What does this mean? If you've been asked to provide a 2:1 equivalent degree, this is the standard UK grading system for degrees. UK degrees are classified out of 100, and are: 1st - 70 or above. 2:1 - 60 to 69. 2:2 - 50 to 59. 3rd - 40 to 49.

Starting at $1/month. Learn more. Web hosting. Starting at $4/month. Learn more. Deploy Now. Try it for free. Learn more. VPS. Starting at $2/month. Learn more. Compute Engine. Try it for free. Learn more. Real solutions for real web pros. That’s code for IONOS. Deploy Now Deploy your code on fast, reliable hosting without leaving GitHub.

Wil jij 2 nachten weg? Profiteer van 50% korting met deze 2=1 aanbiedingen voor je nachtje weg. Ontvang 1 nacht gratis! Boek inclusief gratis ontbijt1 John 2:1. New International Version. 2 My dear children, I write this to you so that you will not sin. But if anybody does sin, we have an advocate with the Father—Jesus Christ, the Righteous One. Read full chapter. 1 John 2:1 in all English translations. 1 John 1.Apr 16, 2024 · The best 2-in-1 laptop we've tested is the ASUS Zenbook 14 Flip OLED (2023). This sleek ultraportable feels incredibly well-built and is easy to carry around. It has a gorgeous 90Hz OLED display, a comfortable keyboard, and a large touchpad with a built-in virtual numpad. The display supports pen input using Microsoft Pen Protocol technology. 2-1/3. This fraction calculator performs basic and advanced fraction operations, expressions with fractions combined with integers, decimals, and mixed numbers. It also shows detailed step-by-step information about the fraction calculation procedure. The calculator helps in finding value from multiple fractions operations. Op zoek naar een 2.1 Speaker? Speakers koop je eenvoudig online bij bol Gratis retourneren 30 dagen bedenktijd Snel in huis. Bekijk alle Speakers!The Zeskit Maya is the best HDMI 2.1 cable for most people. It comes in various sizes from 0.5 meters to 3 meters and has braiding for added durability. Plus, the cable is Ultra High-Speed HDMI-certified. So you know that you can enjoy everything from 4K games at 120Hz and VRR to Dynamic HDR and eARC without issues.We would like to show you a description here but the site won’t allow us.

Google pixel 8a.

Fivi.

Jun 11, 2015 · If you wanted to, you could redirect the stdout and stderr via program 2> log.txt > log.txt or program 2>&1 | cat > log.txt, both of which would have the same effect as using &>. If you do something like program 2>&1 > file, only stdout will be redirected, but stderr can still be piped to another program, such as cat, which could be redirected ... Wrap for 2 hours. After the initial 3-hour smoke, foil the ribs with a liquid of your choosing (I used apple cider, dark brown sugar, and butter) and cook at 225-250 degrees F for 2 more hours. After the braising period, Iook for great retraction of the meat from the bones. You want the bones to be exposed on the end at least 1/4 to 1/2 an inch.换言之 2>1 代表将 stderr 重定向到当前路径下文件名为 1 的 regular file 中,而 2>&1 代表将 stderr 重定向到 文件描述符 为 1 的文件 (即 /dev/stdout )中,这个文件就是 stdout 在 file system 中的映射. 而 &>file 是一种特殊的用法,也可以写成 >&file ,二者的意思完全相 …Jul 12, 2022 · The B.S. goes all the way back to the first two numbers you ever learned: 1 and 2. It turns out these seemingly separate digits are, in fact, exactly the same, and that 1 actually equals 2. And I ... Apr 10, 2024 · The Yoga 9i Gen 8 is a premium swivel-hinge 2-in-1 perfect for anyone who needs touch-input, handwriting, and drawing features but doesn't want to compromise on the laptop experience. The dazzling ... The geometric series on the real line. In mathematics, the infinite series 1 2 + 1 4 + 1 8 + 1 16 + ··· is an elementary example of a geometric series that converges absolutely. The sum of the series is 1. In summation notation, this may be expressed as. The series is related to philosophical questions considered in antiquity, particularly ...2-1/3. This fraction calculator performs basic and advanced fraction operations, expressions with fractions combined with integers, decimals, and mixed numbers. It also shows detailed step-by-step information about the fraction calculation procedure. The calculator helps in finding value from multiple fractions operations.换言之 2>1 代表将 stderr 重定向到当前路径下文件名为 1 的 regular file 中,而 2>&1 代表将 stderr 重定向到 文件描述符 为 1 的文件 (即 /dev/stdout )中,这个文件就是 stdout 在 file system 中的映射. 而 &>file 是一种特殊的用法,也可以写成 >&file ,二者的意思完全相 … ….

1. It comes from that fact that the difference operator Δf(n) = f(n + 1) − f(n) acting on polynomials f(x) of degree d will result in polynomials in degree d − 1 (check this!) - the difference between f(n) = 12 + 22 + ⋯ + n2 and f(n + 1) = 12 + ⋯ + (n + 1)2 is simply (n + 1)2, which is a quadratic in n, hence we should expect f to be ... Algebra. Simplify 2 1/2. 2 1 2 2 1 2. A mixed number is an addition of its whole and fractional parts. 2+ 1 2 2 + 1 2. Add 2 2 and 1 2 1 2. Tap for more steps... 5 2 5 2. The result can be shown in multiple forms..NET Core 2.1 is a Long Term Support (LTS) release and therefore supported for 3 years, or 1 year after the next LTS release ships whichever is longer. .NET Core 2.1 shipped May 30, 2018 and was declared an LTS release with the 2.1.3 update in August 2018. The 3-year lifecycle for the 2.1 release will come to an end on August 21, 2021.2 1 Ontdoe u dus van alles wat slecht is, van alle bedrog en huichelarij, alle afgunst en kwaadsprekerij, 2 en verlang als pasgeboren zuigelingen naar de zuivere melk van het …Op zoek naar een 2.1 Speaker? Speakers koop je eenvoudig online bij bol Gratis retourneren 30 dagen bedenktijd Snel in huis. Bekijk alle Speakers!想定通りですね。そしてcommand > /dev/null 2>&1よりは分かりやすいです。. もう1つは個人的な意見ですが、cronで動かすようなバッチとかは、 標準出力には出すような内容は、ログファイルに出すようにしておくのが良いかと思ってます。Fractions are parts of a whole. Learn how to simplify, add, subtract, multiply and divide fractions with different denominators.Evaluate 1/2-1. Step 1. To write as a fraction with a common denominator, multiply by . Step 2. Combine and . Step 3. Combine the numerators over the common denominator. Step 4. Simplify the numerator. Tap for more steps... Step 4.1. Multiply by . Step 4.2. Subtract from . Step 5. Move the negative in front of the fraction.Sum of the series 2^0 + 2^1 + 2^2 +…..+ 2^n. Given an integer N, the task is to find the sum of series 2 0 + 2 1 + 2 2 + 2 3 + …. + 2 n. A naive approach is to calculate the sum is to add every power of 2 from 0 to n. An efficient approach is to find the 2^ (n+1) and subtract 1 from it since we know that 2^n can be written as: 2 1, Oct 25, 2021 · Ordinary degree (pass): A degree without an honors classification Typically, a Master’s programme will need you have at the very least a Second-class honours, lower division (2.2), sometimes called a lower second class honours. Some do not put an exact grade and request for a ‘good honours degree’ – which usually means a first or a 2.1. , Apr 16, 2023 · A 2-1 buydown is a type of financing that lowers the interest rate on a mortgage for the first two years before it rises to the regular, permanent rate. The rate is typically two percentage points ... , Ephesians 2:1. ESV And you were dead in the trespasses and sins. NIV As for you, you were dead in your transgressions and sins, NASB And you were dead in your offenses and sins, CSB And you were dead in your trespasses and sins. NLT Once you were dead because of your disobedience and your many sins. KJV And you hath he quickened, who …, Jul 12, 2022 · The B.S. goes all the way back to the first two numbers you ever learned: 1 and 2. It turns out these seemingly separate digits are, in fact, exactly the same, and that 1 actually equals 2. And I ... , 9 ou 1 ? On fait le point sur ce calcul devenu viral.Illustrations : Chloé Bouchaour https://www.instagram.com/chloescope_/Sources :Cajori Florian - …, 2.1 Scores. 2.1.1 Complete; 2.1.2 Selections; 2.2 Arrangements and Transcriptions. 2.2.1 Complete. 2.2.1.1 For Orchestra (Brandes) 2.2.1.2 For Strings (Choe) 2.2.1.3 For 2 Violins, Viola and Cello (Brand) 2.2.1.4 For 2 Pianos (Besseling) 2.2.1.5 For 2 Pianos (Bouillet) 2.2.1.6 For Piano 4 hands (Köhler) 2.2.1.7 For Piano 4 Hands (Markull) 2.2. ..., 1=2: A Proof using Beginning Algebra. Step 1: Let a = b . Step 6: and . Step 8: and cancelling the from both sides gives 1=2. See if you can figure out in which step the fallacy lies. When you think you've figured it out, click on that step and the computer will tell you whether you are correct or not, and will give an additional explanation of ..., Don’t take our word for it. The independent DNS monitor DNSPerf ranks 1.1.1.1 the fastest DNS service in the world. Since nearly everything you do on the Internet starts with a DNS request, choosing the fastest DNS directory across all your devices will accelerate almost everything you do online. October, 2020., Op zoek naar een 2.1 Speaker? Speakers koop je eenvoudig online bij bol Gratis retourneren 30 dagen bedenktijd Snel in huis. Bekijk alle Speakers!, To ensure coverage continues, if you're eligible, report changes (contact information, pregnancy, etc.) as soon as possible. Log into your account at yourtexasbenefits.com or call 2-1-1 and select, Option 2. End of Continuous Medicaid Coverage | Texas Health and Human Services. For information about COVID-19, call 2-1-1, Option 1., Mar 27, 2024 · Best budget. Packed with a sleek, flexible design and a decent-performing AMD chipset, the Asus Vivobook S 14 Flip OLED is the best budget 2-in-1 laptop on the market. Aside from its ultraportable ... , Rules for expressions with fractions: Fractions - use a forward slash to divide the numerator by the denominator, i.e., for five-hundredths, enter 5/100.If you use mixed numbers, leave a space between the whole and fraction parts. Mixed numerals (mixed numbers or fractions) keep one space between the integer and fraction and use a forward slash to input …, 2-1-1. 2-1-1 is a special abbreviated telephone number reserved in the North American Numbering Plan (NANP) as an easy-to-remember three-digit code to reach information and referral services to health, human, and social service organizations. Like the emergency telephone number 911, 2-1-1 is one of the eight N11 codes of the North American ... , Rules for expressions with fractions: Fractions - use a forward slash to divide the numerator by the denominator, i.e., for five-hundredths, enter 5/100.If you use mixed numbers, leave a space between the whole and fraction parts. Mixed numerals (mixed numbers or fractions) keep one space between the integer and fraction and use a forward slash to input …, Rules for expressions with fractions: Fractions - use a forward slash to divide the numerator by the denominator, i.e., for five-hundredths, enter 5/100.If you use mixed numbers, leave a space between the whole and fraction parts. Mixed numerals (mixed numbers or fractions) keep one space between the integer and fraction and use a forward slash to input …, Compute answers using Wolfram's breakthrough technology & knowledgebase, relied on by millions of students & professionals. For math, science, nutrition, history ..., Rules for expressions with fractions: Fractions - use a forward slash to divide the numerator by the denominator, i.e., for five-hundredths, enter 5/100.If you use mixed numbers, leave a space between the whole and fraction parts. Mixed numerals (mixed numbers or fractions) keep one space between the integer and fraction and use a forward slash to input …, Free graphing calculator instantly graphs your math problems., 2·1 Architecten staat voor: twee-eenheid tussen opdrachtgever en architect. maatwerk, kwaliteit en betrouwbaarheid. duurzame nieuwbouw, uitbreiding en/of verbouwing. zowel …, The Algebra 1 course, often taught in the 9th grade, covers Linear equations, inequalities, functions, and graphs; Systems of equations and inequalities; Extension of the concept of a function; Exponential models; and Quadratic equations, functions, and graphs. Khan Academy's Algebra 1 course is built to deliver a comprehensive, illuminating, engaging, …, Step 1- Make the rub. In a small bowl, mix the rub ingredients–including garlic and onion powders, brown sugar, smoked paprika, chili powder, and sea salt. Step 2- Rub. Rub mayonnaise all over the ribs, bone side down. Step 3- Season. Season the ribs with the dry rub on both sides., 2 Petrus 1. 1 Van Simeon Petrus, dienaar en apostel van Jezus Christus. Aan allen die dankzij de rechtvaardigheid van onze God en van onze redder Jezus Christus hetzelfde …, What is 6÷2(1+2) = ? This problem went viral and generated millions of comments on Facebook, Twitter, YouTube and social media sites. I explain how to get th..., 2-1-1 is a free and confidential service that helps people across North America find the local resources they need. We’re here for you 24 hours a day, 7 days a week. Home; About 2-1-1; Working with 2-1-1; Give/Volunteer; Resources for Families; Stress Relief; California 2-1-1 Get Connected. Get Answers., 2.1 The Ottoman Empire faced pressure.... 2.2 The 13th Century was the turning point.... HOWEVER, the Section titles themselves aren't numbered, or a number list. I've figured out how to make a number list format that goes 1.1, 1.2, etc. And I've even got a way to make the paragraphs number as 1.1, 1.2, and then 2.1, 2.2 IF they are part of a ..., 2>&1 is an I/O redirection operator used in Bash that redirects the stderr stream to the same destination as the stdout stream. In other words, it merges the error …, Ephesians 2:1. ESV And you were dead in the trespasses and sins. NIV As for you, you were dead in your transgressions and sins, NASB And you were dead in your offenses and sins, CSB And you were dead in your trespasses and sins. NLT Once you were dead because of your disobedience and your many sins. KJV And you hath he quickened, who …, 2-1-1. 2-1-1 is a special abbreviated telephone number reserved in the North American Numbering Plan (NANP) as an easy-to-remember three-digit code to reach information and referral services to health, human, and social service organizations. Like the emergency telephone number 911, 2-1-1 is one of the eight N11 codes of the North American ..., Quick look. $209.99$20999. HP 2023 Premium 2-in-1 Convertible Laptop, 11" HD IPS Touchscreen, Intel 4-Core Pentium Processor Up to 3.30GHz, 8GB Ram, 128GB SSD, Super-Fast 6th Gen WiFi, HDMI, Windows 11 (Renewed) 4.0 out of 5 s. Shop a wide selection of 2 in 1 Laptops at Amazon.com. Free shipping and free returns on eligible items. , Ephesians 2:1. ESV And you were dead in the trespasses and sins. NIV As for you, you were dead in your transgressions and sins, NASB And you were dead in your offenses and sins, CSB And you were dead in your trespasses and sins. NLT Once you were dead because of your disobedience and your many sins. KJV And you hath he quickened, who …, We offer all sorts of two-player games including 1 v 1 Fighting Games, work together in two-player Co-op Games, play with 2 or more players in our Board Games, play Basketball, Soccer, Football and more in our Sports Games, or discover dozens of other games. Examples of popular two player games we have are TicTacToe, Master Chess and …, One half is a rational number that lies midway between nil and unity (which are the elementary additive and multiplicative identities) as the quotient of the first two non-zero integers, . It has two different decimal representations in base ten, the familiar and the recurring , with a similar pair of expansions in any even base; while in odd ..., 那么现在就容易理解前面的疑问了,2>&1表明将文件描述2(标准错误输出)的内容重定向到文件描述符1(标准输出),为什么1前面需要&?当没有&时,1会被认为是一个普通的文件,有&表示重定向的目标不是一个文件,而是一个文件描述符。