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 A007531 a(n) = n*(n-1)*(n-2) (or n!/(n-3)!). (Formerly M4159) 76
 0, 0, 0, 6, 24, 60, 120, 210, 336, 504, 720, 990, 1320, 1716, 2184, 2730, 3360, 4080, 4896, 5814, 6840, 7980, 9240, 10626, 12144, 13800, 15600, 17550, 19656, 21924, 24360, 26970, 29760, 32736, 35904, 39270, 42840, 46620, 50616, 54834, 59280, 63960, 68880 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,4 COMMENTS Ed Pegg Jr conjectures that n^3 - n = k! has a solution if and only if n is 2, 3, 5 or 9 (when k is 3, 4, 5 and 6). Three-dimensional promic (or oblong) numbers, cf. A002378. - Alexandre Wajnberg, Dec 29 2005 Doubled first differences of tritriangular numbers A050534(n) = (1/8)n(n + 1)(n - 1)(n - 2). a(n) = 2*(A050534(n+1) - A050534(n)). - Alexander Adamchuk, Apr 11 2006 If Y is a 4-subset of an n-set X then, for n >= 6, a(n-4) is the number of (n-5)-subsets of X having exactly two elements in common with Y. - Milan Janjic, Dec 28 2007 Convolution of A005843 with A008585. - Reinhard Zumkeller, Mar 07 2009 a(n) = A000578(n) - A000567(n). - Reinhard Zumkeller, Sep 18 2009 For n > 3: a(n) = A173333(n, n-3). - Reinhard Zumkeller, Feb 19 2010 Let H be the n X n Hilbert matrix H(i, j) = 1/(i+j-1) for 1 <= i, j <= n. Let B be the inverse matrix of H. The sum of the elements in row 2 of B equals (-1)^n a(n+1). - T. D. Noe, May 01 2011 a(n) equals 2^(n-1) times the coefficient of log(3) in 2F1(n-2, n-2, n, -2). - John M. Campbell, Jul 16 2011 For n > 2 a(n) = 1/(Integral_{x = 0..Pi/2} (sin(x))^5*(cos(x))^(2*n-5)). - Francesco Daddi, Aug 02 2011 a(n) is the number of functions f: -> [n] that are injective since there are n choices for f(1), (n-1) choices for f(2), and (n-2) choices for f(3). Also, a(n+1) is the number of functions f: -> [n] that are width-2 restricted (that is, the pre-image under f of any element in [n] is of size 2 or less). See "Width-restricted finite functions" link below. - Dennis P. Walsh, Mar 01 2012 This sequence is produced by three consecutive triangular numbers t(n-1), t(n-2) and t(n-3) in the expression 2*t(n-1)*(t(n-2)-t(n-3)) for n = 0, 1, 2, ... - J. M. Bergot, May 14, 2012 For n > 2: A020639(a(n)) = 2; A006530(a(n)) = A093074(n-1). - Reinhard Zumkeller, Jul 04 2012 Number of contact points between equal spheres arranged in a tetrahedron with n - 1 spheres in each edge. - Ignacio Larrosa Cañestro, Jan 07 2013 Also for n >= 3, area of Pythagorean triangle in which one side differs from hypotenuse by two units. Consider any Pythagorean triple (2n, n^2-1, n^2+1) where n > 1. The area of such a Pythagorean triangle is n(n^2-1). For n = 2, 3, 4,.. the areas are 6, 24, 60, .... which are the given terms of the series. - Jayanta Basu, Apr 11 2013 Cf. A130534 for relations to colored forests, disposition of flags on flagpoles, and colorings of the vertices (chromatic polynomial) of the complete graph K_3. - Tom Copeland, Apr 05 2014 a(n)^ Starting with 6, 24, 60, 120, ..., a(n) is the number of permutations of length n>=3 avoiding the partially ordered pattern (POP) {1>2} of length 5. That is, the number of length n permutations having no subsequences of length 5 in which the first element is larger than the second element. - Sergey Kitaev, Dec 11 2020 REFERENCES R. K. Guy, Unsolved Problems in Theory of Numbers, Section D25. L. B. W. Jolley, "Summation of Series", Dover Publications, 1961, p. 40. N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..1000 G. D. Birkhoff, A determinant formula for the number of ways of coloring a map, Ann. Math., 14:42-4. See 1st polynomial p. 5. Alice L. L. Gao, Sergey Kitaev, On partially ordered patterns of length 4 and 5 in permutations, arXiv:1903.08946 [math.CO], 2019. Alice L. L. Gao, Sergey Kitaev, On partially ordered patterns of length 4 and 5 in permutations, The Electronic Journal of Combinatorics 26(3) (2019), P3.26. Milan Janjic, Enumerative Formulas for Some Functions on Finite Sets M. Janjic and B. Petkovic, A Counting Function, arXiv:1301.4550 [math.CO], 2013. Luis Manuel Rivera, Integer sequences and k-commuting permutations, arXiv:1406.3081 [math.CO], 2014-2015. Michelle Rudolph-Lilith, On the Product Representation of Number Sequences, with Application to the Fibonacci Family, arXiv:1508.07894 [math.NT], 2015. Dennis Walsh, Width-restricted finite functions Index entries for linear recurrences with constant coefficients, signature (4,-6,4,-1). FORMULA a(n) = 6*A000292(n-2). a(n) = Sum_{i=1..n} polygorial(3,i) where polygorial(3,i) = A028896(i-1). - Daniel Dockery (peritus(AT)gmail.com), Jun 16 2003 a(n) = 3*a(n-1) - 3*a(n-2) + a(n-3) + 6, n > 2. - Zak Seidov, Feb 09 2006 G.f.: 6*x^2/(1-x)^4. a(-n) = -a(n+2). 1/6 + 3/24 + 5/60 + ... = Sum_{k>=1} (2*k-1)/(k*(k+1)*(k+2)) = 3/4. [Jolley Eq. 213] a(n+1) = n^3 - n. - Mohammad K. Azarian, Jul 26 2007 E.g.f.: x^3*exp(x). - Geoffrey Critzer, Feb 08 2009 If the first 0 is eliminated, a(n) = floor(n^5/(n^2+1)). - Gary Detlefs, Feb 11 2010 1/6 + 1/24 + 1/60 + ... = Sum_{n>=1} 1/(n*(n+1)*(n+2)) = 1/4. - Mohammad K. Azarian, Dec 29 2010 a(0) = 0, a(n) = a(n-1) + 3*(n-1)*(n-2). - Jean-François Alcover, Jan 08 2013 (a(n+1) - a(n))/6 = A000217(n-2) for n > 0. - J. M. Bergot, Jul 30 2013 Partial sums of A028896. - R. J. Mathar, Aug 28 2014 1/6 + 1/24 + 1/60 + ... + 1/(n*(n+1)*(n+2)) = n*(n+3)/(4*(n+1)*(n+2)). - Christina Steffan, Jul 20 2015 a(n+2)^2 = A005563(n)^3 + A005563(n)^2. - Bruno Berselli, May 03 2018 a(n)*a(n+1) + A000096(n-3)^2 = m^2 (a perfect square), m = ((a(n)+a(n+1))/2)-n. - Ezhilarasu Velayutham, May 21 2019 Sum_{n>=3} (-1)^(n+1)/a(n) = 2*log(2) - 5/4. - Amiram Eldar, Jul 02 2020 MAPLE [seq(6*binomial(n, 3), n=0..41)]; # Zerinvary Lajos, Nov 24 2006 MATHEMATICA Table[n^3 - 3n^2 + 2n, {n, 0, 42}] Table[FactorialPower[n, 3], {n, 0, 42}] (* Arkadiusz Wesolowski, Oct 29 2012 *) PROG (PARI) a(n)=n*(n-1)*(n-2) (MAGMA) [n*(n-1)*(n-2): n in [0..40]]; // Vincenzo Librandi, May 02 2011 (Haskell) a007531 n = product [n-2..n]  -- Reinhard Zumkeller, Jul 04 2012 (Sage) [n*(n-1)*(n-2) for n in range(40)] # G. C. Greubel, Feb 11 2019 CROSSREFS Cf. A002378, A005563, A084939, A084940, A084941, A084942, A084943, A084944. Cf. A007531, A028896. Sequence in context: A086768 A160944 A160936 * A331433 A329119 A258345 Adjacent sequences:  A007528 A007529 A007530 * A007532 A007533 A007534 KEYWORD nonn,easy AUTHOR STATUS approved

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Last modified April 19 09:12 EDT 2021. Contains 343110 sequences. (Running on oeis4.)