OFFSET
1,1
COMMENTS
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
REFERENCES
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
LINKS
T. D. Noe, Table of n, a(n) for n = 1..210
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
MATHEMATICA
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 *)
CROSSREFS
KEYWORD
nonn,easy,nice
AUTHOR
STATUS
approved