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A071571
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Smallest number whose square has exactly 2n+1 divisors.
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4
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1, 2, 4, 8, 6, 32, 64, 12, 256, 512, 24, 2048, 36, 30, 16384, 32768, 96, 72, 262144, 192, 1048576, 2097152, 60, 8388608, 216, 768, 67108864, 288, 1536, 536870912, 1073741824, 120, 576, 8589934592, 6144, 34359738368, 68719476736, 180, 864
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OFFSET
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0,2
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COMMENTS
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Only squares have an odd number of divisors.
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LINKS
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FORMULA
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a(n) <= 2^n, where the equality holds if and only if n=0 or 2n+1 is prime. - Jianing Song, Aug 30 2021
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EXAMPLE
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a(4)=6 because it is the smallest number followed by 10,14,15,16,21,22,... whose squares have 2*4 + 1, i.e., 9 divisors.
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PROG
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(PARI) a(n) = {k = 1; while (numdiv(k^2) != (2*n+1), k++); return (k); } \\ Michel Marcus, Jul 27 2013
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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