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A373908
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Number of compositions of 7*n into parts 2 and 7.
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6
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1, 1, 2, 9, 38, 136, 452, 1495, 5031, 17114, 58282, 198032, 671856, 2278870, 7731892, 26238839, 89047335, 302191369, 1025487338, 3479970844, 11809261583, 40074827170, 135994407483, 461498426696, 1566098800484, 5314568565096, 18035031128780, 61202027710656
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OFFSET
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0,3
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LINKS
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FORMULA
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a(n) = Sum_{k=0..floor(n/2)} binomial(n+5*k,n-2*k).
a(n) = 7*a(n-1) - 20*a(n-2) + 35*a(n-3) - 35*a(n-4) + 21*a(n-5) - 7*a(n-6) + a(n-7).
G.f.: 1/(1 - x - x^2/(1 - x)^6).
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PROG
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(PARI) a(n) = sum(k=0, n\2, binomial(n+5*k, n-2*k));
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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