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A137694
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Numbers k such that 6k^2-2k = 3n^2-n for some integer n>0.
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3
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5, 5577, 6435661, 7426747025, 8570459630997, 9890302987423321, 11413401077026881245, 13171054952586033533217, 15199386001883205670450981, 17540078275118266757666898665, 20241235130100477955141930608237, 23358367800057676441967030255006641
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OFFSET
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1,1
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COMMENTS
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Also indices of pentagonal numbers which are half of some other pentagonal number: see A137693 for more details, comments and links.
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LINKS
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FORMULA
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a(n) = f^{2n-2}(5,7)[1], where f(x,y) = (577x + 408y - 164, 816x + 577y - 232).
a(n) = (5,7,1,5,7,1,...) (mod 10).
G.f.: -x*(5-198*x+x^2) / ( (x-1)*(x^2-1154*x+1) ). - R. J. Mathar, Apr 17 2011
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PROG
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(PARI) vector(20, i, (v=if(i>1, [577, 408; 816, 577]*v-[164; 232], [5; 7]))[1, 1])
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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STATUS
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approved
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