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 A006230 Bitriangular permutations. (Formerly M4902) 2
 1, 13, 73, 301, 1081, 3613, 11593, 36301, 111961, 342013, 1038313, 3139501, 9467641, 28501213, 85700233, 257493901, 773268121, 2321377213, 6967277353, 20908123501, 62736953401, 188236026013, 564758409673, 1694375892301, 5083329003481, 15250389663613 (list; graph; refs; listen; history; text; internal format)
 OFFSET 4,2 REFERENCES N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS Colin Barker, Table of n, a(n) for n = 4..1000 Irving Kaplansky and John Riordan, The problem of the rooks and its applications, Duke Mathematical Journal 13.2 (1946): 259-268. Simon Plouffe, Approximations de séries génératrices et quelques conjectures, Dissertation, Université du Québec à Montréal, 1992. Simon Plouffe, 1031 Generating Functions and Conjectures, Université du Québec à Montréal, 1992. J. Riordan, Letter to N. J. A. Sloane, Dec. 1976 Index entries for linear recurrences with constant coefficients, signature (6,-11,6). FORMULA a(n) = 12*S(n-2) + 1, with S(n)=A000392(n) the Stirling numbers of second kind, 3rd column. - Ralf Stephan, Jul 07 2003 a(n+3) = Sum_{i=1..3} A008277(n,i) * A008277(3,i) * i!^2. - Brian Parsonnet, Feb 25 2011 From Colin Barker, Dec 27 2017: (Start) G.f.: x^4*(1 + x)*(1 + 6*x) / ((1 - x)*(1 - 2*x)*(1 - 3*x)). a(n) = 12*(3 - 3*2^(n-2) + 3^(n-2))/6 + 1. a(n) = 6*a(n-1) - 11*a(n-2) + 6*a(n-3) for n>6 (End) MAPLE A006230:=-(z+1)*(6*z+1)/(z-1)/(3*z-1)/(2*z-1); # Conjectured by Simon Plouffe in his 1992 dissertation. MATHEMATICA 12*StirlingS2[n+1, 3]+1; (* Brian Parsonnet, Feb 25 2011 *) Sum[ StirlingS2[n, i] * StirlingS2[ 3, i ] * i!^2, {i, 3} ]; (* alternative, Brian Parsonnet, Feb 25 2011 *) PROG (PARI) Vec(x^4*(1 + x)*(1 + 6*x) / ((1 - x)*(1 - 2*x)*(1 - 3*x)) + O(x^40)) \\ Colin Barker, Dec 27 2017 CROSSREFS Cf. A136301 (row 4). Sequence in context: A081586 A143008 A107963 * A066110 A020527 A146618 Adjacent sequences:  A006227 A006228 A006229 * A006231 A006232 A006233 KEYWORD nonn,easy AUTHOR STATUS approved

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Last modified January 16 03:02 EST 2019. Contains 319184 sequences. (Running on oeis4.)