A086668 - OEIS

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A086668

Number of divisors d of n such 2d+1 is a prime.

1, 2, 2, 2, 2, 4, 1, 3, 3, 3, 2, 4, 1, 3, 4, 3, 1, 6, 1, 4, 3, 3, 2, 5, 2, 3, 3, 3, 2, 7, 1, 3, 4, 2, 3, 7, 1, 2, 3, 5, 2, 6, 1, 4, 5, 3, 1, 6, 1, 4, 3, 3, 2, 7, 3, 5, 2, 3, 1, 8, 1, 2, 5, 3, 3, 6, 1, 3, 4, 5, 1, 8, 1, 3, 5, 2, 2, 7, 1, 5, 4, 3, 2, 6, 2, 3, 3, 5, 2, 10, 1, 3, 2, 2, 3, 7, 1, 4, 6, 5 (list; graph; refs; listen; history; internal format)

	OFFSET	`1,2`
	EXAMPLE	`10 has divisors 1,2,5 and 10 of which 2.1+1, 2.2+1 and 2.5+1 are prime, so a(10)=3`
	PROG	`(PARI) for (n=2, 100, s=0; fordiv(i=n, i, s+=isprime(2*i+1)); print1(", "s))`
	CROSSREFS	`A046886(n) - 1.` `Sequence in context: A119762 A153667 A125914 * A092904 A062816 A122857` `Adjacent sequences: A086665 A086666 A086667 * A086669 A086670 A086671`
	KEYWORD	`nonn`
	AUTHOR	`Jon Perry (perry(AT)globalnet.co.uk), Jul 27 2003`

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Last modified February 14 08:06 EST 2012. Contains 205604 sequences.