A251718 a(n) = smallest positive integer k such that both A083221(k, n) and A083221(k+1, n) have at most two prime factors (are primes or semiprimes).

1, 1, 1, 2, 3, 2, 1, 3, 4, 2, 3, 4, 3, 5, 2, 2, 4, 3, 1, 3, 2, 4, 3, 2, 5, 4, 2, 3, 4, 2, 1, 5, 6, 2, 3, 2, 1, 3, 6, 4, 4, 4, 6, 3, 4, 5, 3, 6, 4, 6, 2, 4, 3, 4, 2, 5, 8, 5, 6, 3, 3, 4, 4, 2, 3, 2, 3, 7, 6, 5, 3, 4, 4, 6, 2, 2, 5, 7, 1, 5, 5, 4, 6, 2, 4, 6, 5, 5, 4, 2, 2, 5, 3, 3, 3, 4, 1, 3, 5, 7, 5, 4, 3, 3, 5, 2, 4, 5, 7, 4, 7, 4, 3, 7, 4, 3, 2, 3, 4, 2

Offset

1,4

Links

Antti Karttunen, Table of n, a(n) for n = 1..509

Formula

For all n, A251717(n) <= a(n) <= A251719(n).

Prog

(Scheme) (define (A251718 n) (let loop ((i 1)) (if (and (<= (A001222 (A083221bi i n)) 2) (<= (A001222 (A083221bi (+ i 1) n)) 2)) i (loop (+ i 1))))) ;; Code for A083221bi given in A083221.

Crossrefs

Variant: A251717.

A005382 gives the positions of 1 after the initial a(1)=1.

Cf. A001222, A083221 (A083140), A251719.

Sequence in context: A117648 A037222 A107357 * A182457 A368605 A334715

Adjacent sequences: A251715 A251716 A251717 * A251719 A251720 A251721

Keyword

nonn

Author

Antti Karttunen, Dec 15 2014

Status

approved