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a(n) = n! + n!! - n - 1.
0

%I #7 Aug 11 2016 17:00:36

%S 1,0,1,5,27,129,761,5137,40695,363815,3632629,39927183,479047667,

%T 6227155921,87178936305,1307676395009,20922800209903,355687462555407,

%U 6402373891522541,121645101063561055,2432902011892531179

%N a(n) = n! + n!! - n - 1.

%t Table[n! + n!! - n - 1, {n, 0, 20}] (* _Bruno Berselli_, Aug 11 2016 *)

%Y Cf. A006882, A110903, A128882.

%K nonn,easy

%O 0,4

%A _Olivier GĂ©rard_, Aug 01 2016