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A111308 Sum_{k=1..n} k!! * (n+1-k)!!, where k!! = k*(k-2)*(k-4)..*(1 or 2). 1

%I

%S 0,1,4,10,28,71,204,556,1716,5049,16884,53418,193500,654639,2560140,

%T 9209016,38697300,147193425,661035060,2645445870,12627252540,

%U 52926443415,267088884300,1167777120420,6200158778100,28178714373705

%N Sum_{k=1..n} k!! * (n+1-k)!!, where k!! = k*(k-2)*(k-4)..*(1 or 2).

%e a(5) = 1 *5*3*1 + 2 *4*2 + 3*1 *3*1 + 4*2 *2 + 5*3*1 *1 = 15 + 16 + 9 + 16 + 15 = 71.

%t Table[ Sum[k!!(n + 1 - k)!!, {k, n}], {n, 25}] (* _Robert G. Wilson v_ *)

%Y Cf. A006882.

%K easy,nonn

%O 0,3

%A _Leroy Quet_, Nov 03 2005

%E More terms from _Robert G. Wilson v_, Nov 04 2005

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Last modified February 28 11:08 EST 2020. Contains 332323 sequences. (Running on oeis4.)