A331399 Numbers k such that A000129(k) and A001333(k) are both prime.

2, 3, 5, 29, 59

Offset

1,1

Comments

This sequence is the intersection of A096650 and A099088.

The k-th square triangular number A001110(k) = (A000129(k)*A001333(k))^2 has exactly 9 divisors iff k is in this sequence, so if a(5) is the final term of this sequence, then there are only 5 triangular numbers that have exactly 9 divisors (cf. A331234).

Links

Table of n, a(n) for n=1..5.

Crossrefs

Cf. A001110 (numbers that are both triangular and square), A000129 (Pell numbers), A001333 (numerators of continued fraction convergents to sqrt(2); equivalently, prime companion Pell numbers, divided by 2), A096650 (indices of prime Pell numbers), A099088 (indices of prime companion Pell numbers, divided by 2), A331234 (triangular numbers having exactly 9 divisors).

Sequence in context: A309179 A019400 A354084 * A084599 A062167 A358896

Adjacent sequences: A331396 A331397 A331398 * A331400 A331401 A331402

Keyword

nonn,more,hard

Author

Jon E. Schoenfield, Jan 16 2020

Status

approved