A078768 sigma(sigma(..(sigma(n)+1)..)) is prime, or 0 if no prime is reached.

1, 0, 1, 0, 1, 1, 0, 0, 0, 1, 1, 1, 0, 0, 0, 0, 1, 3, 6, 1, 5, 1, 0, 1, 0, 1, 1, 6, 1, 1, 5, 0, 4, 2, 4, 6, 7, 1, 6, 21, 1, 2, 2, 1, 1, 4, 2, 3, 3, 1, 2, 2, 7, 1, 7, 5, 2, 1, 5, 3, 1, 2, 0, 2, 2, 2, 1, 1, 2, 1, 4, 3, 3, 2, 2, 1, 5, 5, 34, 1, 2, 4, 1, 3, 7, 1, 2, 4, 1, 5, 3, 2, 7, 4, 2, 10, 1, 2

Offset

1,18

Links

Table of n, a(n) for n=1..98.

Example

a(18)=3 as sigma(18)+1=40, sigma(40)+1=91, sigma(91)+1=113, which is prime

Prog

(PARI) sr(m, c)=if (c>0, m=sigma(m)+1; c--; sr(m, c), m) for (n=1, 100, x=1; while (!isprime(sr(n, x)) && x<200, x++); if (x==200, x=0); print1(x", "))

Crossrefs

Sequence in context: A163327 A338460 A175032 * A307281 A089078 A355072

Adjacent sequences: A078765 A078766 A078767 * A078769 A078770 A078771

Keyword

nonn

Author

Jon Perry, Jan 09 2003

Status

approved