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 A187235 Number of ways to place n nonattacking semi-bishops on an n X n board. 20
 1, 5, 51, 769, 15345, 381065, 11323991, 391861841, 15476988033, 687029386845, 33861652925595, 1834814222811361, 108411291759763681, 6936921762461326545, 477881176664541171375, 35264213540563039871265, 2775185864375851234241985, 232010235620834821000259765, 20534530616200868936398461635 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Two semi-bishops do not attack each other if they are in the same NorthWest-SouthEast diagonal. Conjecture: Number of parity preserving permutations of the set {1, 2, ..., 2n+1} with exactly n+1 cycles (see A246117). - Peter Luschny, Feb 09 2015 LINKS Vaclav Kotesovec, Table of n, a(n) for n = 1..350 V. Kotesovec, Non-attacking chess pieces, 6ed, 2013, p. 260-265. FORMULA a(n)/(n-1)! ~ 0.24252191 * 4.9108149^n where the second constant is 1/(z*(1-z)) = 4.910814964..., where z=0.715331862959... is a root of the equation z=2*(z-1)*log(1-z). For constants see A238261 and A238262. - Vaclav Kotesovec, Feb 21 2014 a(n) = (-1)^n * Sum_{i=0..n} Stirling1(n,i) * Stirling1(n+1,n-i+1). - Ryan Brooks, May 09 2020 MATHEMATICA Table[If[n==1, 1, Coefficient[Expand[Product[x+i, {i, 1, n}]*Product[x+i, {i, 1, n-1}], x], x, n-1]], {n, 1, 50}] Table[(-1)^n*Sum[StirlingS1[n+1, j]*StirlingS1[n, n-j+1], {j, 1, n}], {n, 1, 50}] (* Explicit formula, Vaclav Kotesovec, Mar 24 2011 *) PROG (PARI) a(n) = {(-1)^n*sum(i=0, n, stirling(n, i, 1) * stirling(n+1, n-i+1, 1))} \\ Andrew Howroyd, May 09 2020 CROSSREFS Cf. A238261, A238262, A002465, A099152, A000255, A129256. Sequence in context: A154886 A268138 A145162 * A318192 A299435 A095839 Adjacent sequences:  A187232 A187233 A187234 * A187236 A187237 A187238 KEYWORD nonn,nice AUTHOR Vaclav Kotesovec, Mar 08 2011 STATUS approved

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Last modified November 25 11:32 EST 2020. Contains 338623 sequences. (Running on oeis4.)