A092062 Numbers k such that A061015(k) is prime.

2, 10, 18, 36, 90, 759

Offset

1,1

Comments

a(6) > 447 for a(6) the numerator has more than 2673 digits.

a(7) > 1850. - Michael S. Branicky, Jun 27 2022

Links

Table of n, a(n) for n=1..6.

Formula

Numbers k such that numerator of (Sum_{i=1..k} 1/prime(i)^2) is prime

Example

1/2^2 = 1/4 but 1 is not prime, 1/2^2 + 1/3^2 = 13/36 and 13 is prime so a(1)=2.

Prog

(PARI) sm(n)= s=0; for(i=1, n, s=s+1/(prime(i)^2)); return(s);
for (i=1, 400, if(isprime(numerator(sm(i))), print1(i, ", ")))
(Python) # uses A061015gen() and imports from A061015
from sympy import isprime
def agen():
    yield from (k for k, ak in enumerate(A061015gen(), 1) if isprime(ak))
print(list(islice(agen(), 5))) # Michael S. Branicky, Jun 27 2022

Crossrefs

Cf. A061015.

Sequence in context: A082969 A173592 A018227 * A271786 A134251 A317714

Adjacent sequences: A092059 A092060 A092061 * A092063 A092064 A092065

Keyword

hard,nonn

Author

Mohammed Bouayoun (mohammed.bouayoun(AT)sanef.com), Feb 20 2004

Extensions

a(6) from Alexander Adamchuk, Sep 16 2010

Status

approved