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A051055 'Connected' alternating sign n X n matrices, i.e., not made from smaller blocks.

%I A051055 #26 Aug 03 2017 02:53:15
%S A051055 0,1,0,1,2,59,1092,51412,3420384,382912420,68021283668,19474443244283,
%T A051055 9025228384142396,6825775070789988992,8486240219059861120000,
%U A051055 17454179683586670023001218,59698062960218238908531091872
%N A051055 'Connected' alternating sign n X n matrices, i.e., not made from smaller blocks.
%C A051055 A003827 factors out the singleton components only, but many alternating sign matrices can be decomposed into larger pieces.
%H A051055 Alois P. Heinz, <a href="/A051055/b051055.txt">Table of n, a(n) for n = 0..90</a>
%F A051055 Sum_{k>=0} a(k)z^k/k!^2 = log(Sum_{k>=0} r(k)z^k/k!^2) where r(k) is the k-th Robbins number A005130(n).
%F A051055 a(n) = r(n) - (1/n)*Sum_{k=0..n-1} k*binomial(n, k)^2*r(n-k)*a(k), n > 0, a(0)=0, where r(k) is the k-th Robbins number A005130(n). - _Vladeta Jovovic_, Mar 16 2000
%e A051055 a(4)=2 because of the alternating sign matrices {{0,1,0,0},{1,-1,1,0},{0,1,-1,1},{0,0,1,0}} and {{0,0,1,0},{0,1,-1,1},{1,-1,1,0},{0,1,0,0}}.
%t A051055 r[n_] = Product[(3k+1)!/(n+k)!, {k, 0, n-1}] ; a[n_] := a[n] = r[n] - (1/n)*Sum[k*Binomial[n, k]^2*r[n-k]*a[k], {k, 0, n-1}]; a[0] = 0; Table[a[n], {n, 0, 16}] (* _Jean-François Alcover_, Aug 01 2011, after _Vladeta Jovovic_ *)
%Y A051055 Cf. A003827, A005130.
%K A051055 nice,easy,nonn
%O A051055 0,5
%A A051055 _Don Knuth_
%E A051055 More terms from _Vladeta Jovovic_, Mar 16 2000