A155874 - OEIS

A155874

Smallest positive composite number such that a(n)+n is also composite.

4, 8, 4, 6, 4, 4, 4, 8, 4, 6, 4, 4, 4, 8, 4, 6, 4, 4, 4, 6, 4, 4, 4, 4, 4, 8, 4, 6, 4, 4, 4, 4, 4, 6, 4, 4, 4, 8, 4, 6, 4, 4, 4, 6, 4, 4, 4, 4, 4, 6, 4, 4, 4, 4, 4, 8, 4, 6, 4, 4, 4, 4, 4, 6, 4, 4, 4, 8, 4, 6, 4, 4, 4, 4, 4, 6, 4, 4, 4, 6, 4, 4, 4, 4, 4, 6, 4, 4, 4, 4, 4, 4, 4, 6, 4, 4, 4, 8, 4, 6, 4

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OFFSET

0,1

COMMENTS

a(0)=4. Adding a(0) to n=0 gives 4+0=4, which is a composite number; adding a(1) to n=1 gives 8+1=9 which is composite; adding a(2) to n=2 gives 4+2=6 which is composite; adding a(3) to n=3 gives 6+3=9 which is composite; etc.

At least one of {n+4, n+6, n+8} is divisible by 3, so a(n) is in {4,6,8} for all n. - Charlie Neder, Dec 28 2018

LINKS

Charlie Neder, Table of n, a(n) for n = 0..1000

FORMULA

If n+4 is in A001359, a(n) = 8. If n+4 is in A067774, a(n) = 6. Otherwise, a(n) = 4. - Charlie Neder, Dec 28 2018

PROG

(PARI) a(n) = {forcomposite(k=4, 10, if (!isprime(k+n), return(k)); ); } \\ Michel Marcus, Dec 28 2018

CROSSREFS

Cf. A122984.

Sequence in context: A122110 A082632 A296481 * A160204 A200392 A195289

Adjacent sequences: A155871 A155872 A155873 * A155875 A155876 A155877

KEYWORD