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A136327
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Numbers k such that binomial(2k-1, k-1) == 1 (mod k).
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4
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2, 3, 5, 7, 9, 11, 13, 17, 19, 23, 25, 29, 31, 37, 41, 43, 47, 49, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 121, 125, 127, 131, 137, 139, 149, 151, 157, 163, 167, 169, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241, 251
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OFFSET
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1,1
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COMMENTS
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Contains primes, squares of odd primes and cubes of primes >= 5.
See A228562 for terms that are neither primes nor prime powers. [Joerg Arndt, Aug 27 2013]
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LINKS
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EXAMPLE
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a(3) = 5 because binomial(9, 4) = 126 == 1 (mod 5).
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MATHEMATICA
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Select[Range[300], Mod[Binomial[2# - 1, # - 1], #] == 1 &] (* Alonso del Arte, May 11 2014 *)
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PROG
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(PARI) isok(n) = (binomial(2*n-1, n-1) % n) == 1; \\ Michel Marcus, Aug 26 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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STATUS
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approved
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