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 A370286 Coefficient of x^n in the expansion of ( (1+x)^3 + x^2 )^n. 1
 1, 3, 17, 102, 645, 4193, 27764, 186231, 1261213, 8604759, 59053167, 407217396, 2819252544, 19583729766, 136426565999, 952743556907, 6667916884701, 46755146944959, 328398159653117, 2310073990369926, 16271915501598595, 114757849228310355 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 LINKS Table of n, a(n) for n=0..21. FORMULA a(n) = Sum_{k=0..floor(n/2)} binomial(n,k) * binomial(3*n-3*k,n-2*k). The g.f. exp( Sum_{k>=1} a(k) * x^k/k ) has integer coefficients and equals (1/x) * Series_Reversion( x / ((1+x)^3 + x^2) ). PROG (PARI) a(n) = sum(k=0, n\2, binomial(n, k)*binomial(3*n-3*k, n-2*k)); CROSSREFS Cf. A002426, A006139. Cf. A188686, A370287. Cf. A065065. Sequence in context: A074565 A339565 A241768 * A054365 A116886 A163064 Adjacent sequences: A370283 A370284 A370285 * A370287 A370288 A370289 KEYWORD nonn AUTHOR Seiichi Manyama, Feb 14 2024 STATUS approved

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Last modified June 21 09:49 EDT 2024. Contains 373543 sequences. (Running on oeis4.)