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A257925
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a(n) = (n^2 - n + 1)*(n^2 + n - 1).
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0
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1, 15, 77, 247, 609, 1271, 2365, 4047, 6497, 9919, 14541, 20615, 28417, 38247, 50429, 65311, 83265, 104687, 129997, 159639, 194081, 233815, 279357, 331247, 390049, 456351, 530765, 613927, 706497, 809159, 922621
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OFFSET
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1,2
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COMMENTS
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Subsequence of a(m,n)=(m^2 + n).(n^2 + m)/(m - n)^3 with m=n-1. Q N4 of the 2012 International Mathematical Olympiad paper poses the problem of proving more than 500 solutions exist below 2012 for the equation: a(m,n).(m - n)^3=(m^2 + n).(n^2 + m). Such solutions a(m,n) were called 'Friendly'. If m=2k-1 and n=k-1, solutions of the form a=4k-3 for some integer k, satisfy this requirement although others do exist for other (m,n) pairs e.g. if (m,n)=(1,2), a(m,n)=15.
If m=n-2, a(n)=(n^2 - 3*n + 4)*(n^2 + n - 2)/8. This is the sequence A176145 [t*(t-3)*(t^2-7*t+14)/8] with t=n+2.
Satisfies a linear recurrence having signature (5, -10, 10, -5, 1). - Harvey P. Dale, Apr 18 2019
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LINKS
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FORMULA
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a(n) = (n^2 - n + 1)*(n^2 + n - 1).
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EXAMPLE
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For n=1, a(1) = 1;
For n=2, a(2) = 15;
For n=3, a(3) = 77.
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MATHEMATICA
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PROG
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(PARI) a(n) = (n^2 - n + 1)*(n^2 + n - 1); \\ Michel Marcus, Apr 17 2016
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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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