

A078768


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


0



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
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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: A181099 A163327 A175032 * A089078 A134804 A145389
Adjacent sequences: A078765 A078766 A078767 * A078769 A078770 A078771


KEYWORD

nonn


AUTHOR

Jon Perry, Jan 09 2003


STATUS

approved



