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A086668 Number of divisors d of n such that 2d+1 is a prime. 4
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; text; internal format)
OFFSET

1,2

COMMENTS

From Antti Karttunen, Jun 15 2018: (Start)

Number of terms of A005097 that divide n.

For all n >= 1, a(n) > A156660(n). Specifically, a(p) = 2 for all p in A005384 (Sophie Germain primes), although 2's occur in other positions as well.

(End)

LINKS

Antti Karttunen, Table of n, a(n) for n = 1..65537

FORMULA

From Antti Karttunen, Jun 15 2018: (Start)

a(n) = Sum_{d|n} A101264(d).

a(n) = A305818(n) + A101264(n).

(End)

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

MATHEMATICA

Table[Count[Divisors[n], _?(PrimeQ[2#+1]&)], {n, 100}] (* Harvey P. Dale, Apr 29 2015 *)

PROG

(PARI) for (n=2, 100, s=0; fordiv(i=n, i, s+=isprime(2*i+1)); print1(", "s))

(PARI) A086668(n) = sumdiv(n, d, isprime(d+d+1)); \\ Antti Karttunen, Jun 15 2018

CROSSREFS

One less than A046886.

Cf. A005097, A005384, A101264, A305818.

Sequence in context: A216322 A335383 A125914 * A092904 A231883 A062816

Adjacent sequences:  A086665 A086666 A086667 * A086669 A086670 A086671

KEYWORD

nonn

AUTHOR

Jon Perry, Jul 27 2003

EXTENSIONS

Definition modified by Harvey P. Dale, Apr 29 2015

STATUS

approved

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Last modified October 1 15:55 EDT 2020. Contains 337443 sequences. (Running on oeis4.)