%I #7 Jan 11 2018 00:10:31
%S 0,0,1,1,4,16,85,533,3888,32268,300193,3093377,34970468,430293488,
%T 5724411845,81871668773,1252691817856,20417647146396,353169795861921,
%U 6461418976274657,124664729763630436
%N a(0)=a(1)=0; thereafter a(n) = A162973(n)/(n-1).
%H John Riordan, <a href="/A002720/a002720_3.pdf">Letter to N. J. A. Sloane, Sep 26 1980 with notes on the 1973 Handbook of Integer Sequences</a>. Note that the sequences are identified by their N-numbers, not their A-numbers.
%F Riordan gives a recurrence.
%K nonn
%O 0,5
%A _N. J. A. Sloane_, Jan 10 2018