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 A136112 Indices of pentagonal numbers > 0 which are not the difference of two larger pentagonal numbers. 12
 1, 2, 3, 5, 6, 8, 9, 11, 15, 18, 23, 24, 27, 51, 54, 71, 72, 81, 96, 99, 123, 128, 135, 162, 216, 239, 243, 263, 288, 303, 311, 359, 384, 423, 459, 479, 486, 519, 591, 599, 639, 648, 683, 699, 729, 743, 783, 863, 864, 879, 891, 911, 1031, 1103, 1151, 1215, 1431 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS Donovan Johnson, Table of n, a(n) for n = 1..500 FORMULA A number m is in this sequence iff A136114(m) = 0 iff A136115(m) = 0 EXAMPLE a(1..3)=1,2,3 since P(1),P(2),P(3) cannot be written as difference of 2 other pentagonal numbers. P(4)=22=P(8)-P(7), therefore 4 is not in this sequence. PROG (PARI) P(n)=n*(3*n-1)>>1 isPent(t)=P(sqrtint((t<<1)\3)+1)==t for( i=1, 999, for( j=i+1, (P(i)-1)\3, isPent(P(i)+P(j))&next(2)); print1(i", ")) CROSSREFS Cf. A000326, A136113-A136118. Sequence in context: A219729 A335659 A000534 * A127936 A280771 A348195 Adjacent sequences:  A136109 A136110 A136111 * A136113 A136114 A136115 KEYWORD nonn AUTHOR M. F. Hasler, Dec 15 2007 EXTENSIONS Definition corrected, thanks to a remark from R. J. Mathar, Feb 01 2008 More terms from Donovan Johnson, Apr 22 2008 STATUS approved

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Last modified August 11 20:13 EDT 2022. Contains 356067 sequences. (Running on oeis4.)