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A155874
Smallest positive composite number such that a(n)+n is also composite.
3
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
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
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
KEYWORD
easy,nonn
AUTHOR
Eric Angelini, Jan 29 2009
EXTENSIONS
More terms from Charlie Neder, Dec 28 2018
STATUS
approved