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A140802
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a(n) = binomial(n+3, 3)*8^n.
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9
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1, 32, 640, 10240, 143360, 1835008, 22020096, 251658240, 2768240640, 29527900160, 307090161664, 3126736191488, 31267361914880, 307863255777280, 2990671627550720, 28710447624486912, 272749252432625664, 2567051787601182720, 23959150017611038720
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OFFSET
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0,2
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COMMENTS
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With a different offset, number of n-permutations (n>=3) of 9 objects: r, s, t, u, v, w, z, x, y with repetition allowed, containing exactly (3) three u's.
Example:
(n=4) a(1)=32
uuur, uuru, uruu, ruuu,
uuus, uusu, usuu, suuu,
uuut, uutu, utuu, tuuu,
uuuv, uuvu, uvuu, vuuu,
uuuw, uuwu, uwuu, wuuu,
uuuz, uuzu, uzuu, zuuu,
uuux, uuxu, uxuu, xuuu,
uuuy, uuyu, uyuu, yuuu
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LINKS
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FORMULA
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Sum_{n>=0} 1/a(n) = 1176*log(8/7) - 156.
Sum_{n>=0} (-1)^n/a(n) = 1944*log(9/8) - 228. (End)
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MAPLE
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seq(binomial(n+3, 3)*8^n, n=0..19);
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MATHEMATICA
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nn = 21; Drop[Range[0, nn]!CoefficientList[Series[x^3/3! Exp[8x], {x, 0, nn}], x], 3] (* Geoffrey Critzer, Oct 03 2013 *)
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PROG
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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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