A371785 a(n) = Sum_{k=0..floor(n/2)} (-1)^k * binomial(3*n-k,n-2*k).
1, 3, 14, 76, 441, 2652, 16303, 101727, 641630, 4080154, 26112384, 167978615, 1085182436, 7035477777, 45750406205, 298279844724, 1949096816505, 12761551428024, 83701819019155, 549850618355886, 3617119500327536, 23824816811652905, 157106267803712709
Offset: 0
Keywords
Programs
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PARI
a(n) = sum(k=0, n\2, (-1)^k*binomial(3*n-k, n-2*k));
Formula
a(n) = [x^n] 1/((1-x+x^2) * (1-x)^(2*n)).