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A085102
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Go on adding the divisors of n starting from n in decreasing order until one gets a prime. a(n) = this prime, or 0 if no prime is obtained.
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1
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0, 2, 3, 7, 5, 11, 7, 0, 13, 17, 11, 0, 13, 23, 23, 31, 17, 0, 19, 41, 31, 0, 23, 59, 31, 41, 0, 53, 29, 61, 31, 0, 47, 53, 47, 0, 37, 59, 0, 83, 41, 0, 43, 83, 0, 71, 47, 0, 0, 0, 71, 97, 53, 0, 71, 113, 79, 89, 59, 137, 61, 0, 103, 127, 83, 0, 67, 0, 0, 0, 71, 179, 73, 113, 0, 137, 0
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OFFSET
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1,2
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COMMENTS
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1. a(p) = p, where p is a prime, by definition. 2. If 2^k -1 is a Mersenne prime then a(2^(k-1)) = 2^k -1 else a(2^(k-1))= 0. 3. a(p^(2k+1)) = 0, if p is prime.
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LINKS
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EXAMPLE
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a(28) = 53 because 28+14+7+4 = 53 is prime.
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PROG
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(PARI) a(n) = {d = divisors(n); p = 0; forstep (i = #d, 1, -1, p += d[i]; if (isprime(p), return (p)); ); return (0); } \\ Michel Marcus, Sep 17 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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Amarnath Murthy and Meenakshi Srikanth (menakan_s(AT)yahoo.com), Jul 03 2003
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EXTENSIONS
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STATUS
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approved
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