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A333012
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Integers which can be written in exactly two ways as a sum of two distinct nonzero pentagonal numbers.
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4
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211, 215, 381, 447, 602, 663, 766, 807, 853, 874, 1002, 1172, 1197, 1248, 1259, 1372, 1427, 1457, 1571, 1612, 1622, 1639, 1652, 1665, 1752, 1862, 1927, 1996, 2047, 2152, 2245, 2297, 2302, 2332, 2351, 2415, 2472, 2497, 2506, 2523, 2618, 2887, 2912, 2952
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OFFSET
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1,1
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LINKS
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EXAMPLE
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211 = P(5) + P(11) = P(1) + P(12) = 35 + 176 = 1 + 210, where P(n) is the n-th pentagonal number A000326.
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PROG
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(PARI) is(k) = sum(i=1, sqrt(1+12*k)\6, sqrt(1+24*k+12*i-36*i*i)%6==5) == 2; \\ Jinyuan Wang, Mar 06 2020
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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