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a(n) = n! - binomial(n,3).
0

%I #13 Aug 03 2020 04:30:07

%S 1,1,2,5,20,110,700,5005,40264,362796,3628680,39916635,479001380,

%T 6227020514,87178290836,1307674367545,20922789887440,355687428095320,

%U 6402373705727184,121645100408831031,2432902008176638860,51090942171709438670,1124000727777607678460

%N a(n) = n! - binomial(n,3).

%t Table[n! - Binomial[n, 3], {n, 20}] (* _David Callan_, May 08 2016 *)

%K nonn,easy

%O 0,3

%A _Roger L. Bagula_, Jun 10 2007

%E Edited by _Joerg Arndt_, Aug 03 2020