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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). 4
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 (list; graph; refs; listen; history; text; internal format)
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 A334715 A026105

Adjacent sequences:  A251715 A251716 A251717 * A251719 A251720 A251721

KEYWORD

nonn

AUTHOR

Antti Karttunen, Dec 15 2014

STATUS

approved

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Last modified January 17 23:51 EST 2022. Contains 350410 sequences. (Running on oeis4.)