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A231507 a(n) is smallest number greater than a(n-1) such that a(n)+a(n-1) is composite. 1
4, 5, 7, 8, 10, 11, 13, 14, 16, 17, 18, 20, 22, 23, 25, 26, 28, 29, 31, 32, 33, 35, 37, 38, 39, 41, 43, 44, 46, 47, 48, 50, 52, 53, 55, 56, 58, 59, 60, 61, 62, 63, 65, 67, 68, 70, 71, 72, 73, 74, 76, 77, 78, 80, 81, 83, 85, 86, 88, 89, 91, 92, 93, 94, 95, 97 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Harvey P. Dale, Table of n, a(n) for n = 1..1000

EXAMPLE

a(1) = 4, the first composite. So the smallest a(2) could possibly be 5. 4+5=9, which is composite, so a(2) = 5. a(3) cannot be 6, because 5+6=11, which is prime. But 5+7=12 is composite, so a(3) = 7.

MATHEMATICA

nxt[n_]:=Module[{k=n+1}, While[PrimeQ[n+k], k++]; k]; NestList[nxt, 4, 70] (* Harvey P. Dale, Jul 18 2014 *)

CROSSREFS

Cf. A062042, A073627.

Sequence in context: A248188 A174047 A285085 * A097482 A191984 A219958

Adjacent sequences:  A231504 A231505 A231506 * A231508 A231509 A231510

KEYWORD

nonn

AUTHOR

Neil Fernandez, Nov 09 2013

STATUS

approved

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Last modified September 15 12:22 EDT 2019. Contains 327078 sequences. (Running on oeis4.)