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A116992
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Number of primes < (highest prime dividing any composite between the n-th and (n+1)th prime) that are coprime to every composite between the n-th and (n+1)th prime.
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1
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0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 1, 3, 1, 4, 1, 3, 0, 4, 3, 0, 4, 9, 6, 6, 0, 4, 10, 0, 6, 4, 9, 11, 6, 10, 0, 2, 15, 17, 6, 16, 0, 5, 0, 19, 2, 13, 14, 25, 5, 3, 13, 0, 12, 23, 23, 15, 0, 24, 28, 12, 12, 20, 20, 3, 31, 22, 31, 27, 7, 0, 32, 32, 7, 6, 37, 36, 34, 40, 14, 20, 0, 33, 0, 19, 0, 40
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OFFSET
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1,12
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LINKS
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EXAMPLE
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Between the 12th prime and the 13th prime are the composites 38, 39 and 40.
Dividing these composites are the primes 2, 3, 5, 13 and 19. There are three primes < 19 and coprime to the composites between 37 and 41: 7, 11 and 17. So a(12) = 3.
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PROG
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(PARI) a(n) = {p = prime(n); q = prime(n+1); vp = []; for (x=p+1, q-1, f = factor(x); for (i=1, #f~, vp = Set(concat(vp, f[i, 1]))); ); if (#vp == 0, return (0)); nb = 0; forprime (pp=2, precprime(vecmax(vp)-1), ok = 1; for (x=p+1, q-1, if (gcd(x, pp) != 1, ok = 0; break; ); ); if (ok, nb++); ); nb; } \\ Michel Marcus, Mar 01 2015
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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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