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fermatslibrary's profile
Fermat's Library
Fermat's Library
Fermat's Library
@fermatslibrary

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Fermat's Library

@fermatslibrary

A platform for illuminating academic papers. We publish an annotated paper every week. Our chrome extension for arXiv: https://fermatslibrary.com/librarian 

fermatslibrary.com
Joined September 2015

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    Fermat's Library‏ @fermatslibrary 28 Nov 2018
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    The subfactorial !n is the number of derangements of n objects. !1=0 !2=1 !3=2 !4=9pic.twitter.com/w9WLOZQSe3

    6:22 AM - 28 Nov 2018
    • 267 Retweets
    • 1,066 Likes
    • 张先波 Abdullah Baran Pavel V., MD PhD Matematickcom James thecalculuscism おかもとかずま Vikram Dutt ハヤセ Adam Naumiec
    23 replies 267 retweets 1,066 likes
      1. New conversation
      2. Tamás Görbe‏ @TamasGorbe 28 Nov 2018
        • Report Tweet
        • Report NetzDG Violation
        Replying to @fermatslibrary

        In case you're wondering a derangement is a permutation that has no fixed points. Exact formula !n=n! ∑ᵢ₌₀ⁿ(-1)ⁱ/i! Recursive formula !n=[n-1][!(n-1)+!(n-2)] with !1=0 and !2=1, And of course, good old @numberphile & @jamesgrime have got you covered!https://youtu.be/pbXg5EI5t4c 

        2 replies 8 retweets 89 likes
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      1. DℝOℕ∈‏ @d_r_o_n_e 28 Nov 2018
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        Replying to @fermatslibrary

        The nth subfactorial is the number of permutations of n objects in which no object appears in its natural place. Long story short: it's alternating series of permutations.pic.twitter.com/2iWzvZJrsw

        0 replies 7 retweets 46 likes
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      1. Franck Lebeau‏ @_Kcnarf 28 Nov 2018
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        Replying to @fermatslibrary

        [inspired from wikipedia] Suppose that a professor gave a test to 4 students – A, B, C, and D – and wants to let them grade each other's tests, but no student should grade his/her own test. Out of 24 possible permutations (4!), there are only 9 derangements (blue italics in fig)pic.twitter.com/P9uwitkq0B

        0 replies 4 retweets 18 likes
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      2. 𝚒𝚗𝚝𝚎𝚛𝚛𝚘𝚋𝚊𝚗𝚐‽‏ @TheAt0miX 28 Nov 2018
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        Replying to @fermatslibrary

        !n ≈ n! / e

        2 replies 3 retweets 15 likes
      3. وائل الحربي- Wael Alharbi 🇸🇦‏ @waelrehaili 28 Nov 2018
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        Replying to @TheAt0miX @fermatslibrary

        Thanks...tested it and it is correct

        0 replies 0 retweets 3 likes
      4. End of conversation
      1. New conversation
      2.  📈atha 📈iel  📊ood 📊ard‏ @Distributino 28 Nov 2018
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        Replying to @fermatslibrary

        Apparently !0=1 but this confuses me

        2 replies 0 retweets 6 likes
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      1. haℤy ben‏ @DavidHdezBen 28 Nov 2018
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        Replying to @fermatslibrary

        haℤy ben Retweeted fun with functions

        https://twitter.com/funfunfunctions/status/999265956378632192 …

        haℤy ben added,

        fun with functions @funfunfunctions
        148.349 es el único número igual a la suma de subfactoriales de sus cifras. Un subfactorial (!n) cuenta el número de formas diferentes en que n amigos pueden intercambiar regalos si cada uno da y recibe un regalo. Por ejemplo, en un amigo invisible con 4 personas, !4 = 9. pic.twitter.com/YDApyteipm
        0 replies 4 retweets 5 likes
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      2. Juan Perez‏ @jperesp 28 Nov 2018
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        Replying to @mistySpock @fermatslibrary

        None, because of the Patronus Charm.

        0 replies 0 retweets 1 like
      3. End of conversation

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