%I A006957
%S A006957 1,2,7,32,185,1310,11067,109148,1234045,15752858,224169407,3518636504,
%T A006957 60381131265,1124390692886,22577494959427,486212633129300,
%U A006957 11177317486573445,273173247028616594,7072436847620016327
%N A006957 Self-convolution of numbers of preferential arrangements.
%p A006957 f:= proc(n) option remember; `if`(n<=1, 1, add (binomial (n, k) *f(n-k), 
               k=1..n)) end: a:= n-> add (f(k) *f(n-k), k=0..n): seq (a(n), n=0..25); 
               [From Alois P. Heinz (heinz(AT)hs-heilbronn.de), Feb 02 2009]
%Y A006957 Cf. A000670.
%Y A006957 Sequence in context: A000153 A006154 A000987 this_sequence A079265 A112006 
               A067551
%Y A006957 Adjacent sequences: A006954 A006955 A006956 this_sequence A006958 A006959 
               A006960
%K A006957 nonn
%O A006957 0,2
%A A006957 N. J. A. Sloane (njas(AT)research.att.com).
%E A006957 More terms from Alois P. Heinz (heinz(AT)hs-heilbronn.de), Feb 02 2009