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A003679
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Numbers that are not the sum of 3 pentagonal numbers.
(Formerly M3323)
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9
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4, 8, 9, 16, 19, 20, 21, 26, 30, 31, 33, 38, 42, 43, 50, 54, 55, 60, 65, 67, 77, 81, 84, 88, 89, 90, 96, 99, 100, 101, 111, 112, 113, 120, 125, 131, 135, 138, 142, 154, 159, 160, 166, 170, 171, 183, 195, 204, 205, 207, 217, 224, 225, 226, 229, 230, 236, 240, 241
(list;
graph;
refs;
listen;
history;
text;
internal format)
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OFFSET
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1,1
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COMMENTS
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Guy's paper says that the sequence probably contains exactly 210 terms, six of which require five pentagonal numbers: 9, 21, 31, 43, 55 and 89. The last term is conjectured to be 33066. - T. D. Noe, Apr 19 2006
The next term, if it exists, is greater than 160000000. - Jack W Grahl, Jul 10 2018
a(211) > 10^11, if it exists. - Giovanni Resta, Jul 13 2018
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REFERENCES
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N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
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T. D. Noe, Table of n, a(n) for n = 1..210
J. W. Grahl, C code which was used to check for elements of this sequence up to 160,000,000.
R. K. Guy, Every number is expressible as the sum of how many polygonal numbers?, Amer. Math. Monthly 101 (1994), 169-172.
Eric Weisstein's World of Mathematics, Pentagonal Number
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MATHEMATICA
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nn=200; pen=Table[n(3n-1)/2, {n, 0, nn-1}]; lst=Range[pen[[ -1]]; Do[n=pen[[i]]+pen[[j]]+pen[[k]]; If[n<=pen[[ -1]], lst=DeleteCases[lst, n]]], {i, nn}, {j, i, nn}, {k, j, nn}]; lst (* T. D. Noe, Apr 19 2006 *)
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CROSSREFS
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Cf. A117065 (primes in this sequence).
Cf. A118278, A118279.
Sequence in context: A034038 A069265 A336359 * A079432 A162215 A134344
Adjacent sequences: A003676 A003677 A003678 * A003680 A003681 A003682
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KEYWORD
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nonn,easy,nice
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AUTHOR
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N. J. A. Sloane, Mira Bernstein
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STATUS
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approved
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