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A340996 a(n) is the number of different primes that can be expressed as n mod p where p < n is prime. 2
0, 0, 0, 0, 1, 0, 1, 2, 1, 1, 1, 2, 2, 2, 1, 3, 2, 3, 2, 3, 1, 3, 2, 5, 2, 4, 2, 4, 3, 4, 3, 3, 4, 3, 1, 6, 3, 4, 2, 6, 3, 6, 3, 5, 4, 5, 4, 7, 4, 6, 4, 5, 3, 8, 3, 5, 3, 6, 4, 9, 3, 6, 5, 8, 4, 7, 2, 6, 4, 8, 4, 9, 5, 7, 5, 8, 3, 9, 5, 7, 7, 7, 4, 10, 5, 6, 5, 7, 6, 12, 5, 7, 7, 7, 4, 11, 5, 9 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,8
COMMENTS
a(n) is the number of primes p < n such that n-p has a prime factor > p.
a(n) <= A056172(n-1), with equality for n = 1, 2, 3, 4, 5, 8, 24.
LINKS
EXAMPLE
a(8) = 2 because 2 = 8 mod 3 and 3 = 8 mod 5.
MAPLE
f:= proc(n) local k; nops(select(isprime, {seq(n mod k, k=select(isprime, [$2..n-1]))})) end proc:
map(f, [$1..100]);
CROSSREFS
Sequence in context: A087048 A109700 A087742 * A072530 A184341 A090455
KEYWORD
nonn,look
AUTHOR
J. M. Bergot and Robert Israel, Feb 01 2021
STATUS
approved

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Last modified April 16 17:36 EDT 2024. Contains 371749 sequences. (Running on oeis4.)