OFFSET
1,3
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
EXAMPLE
a(3) = 2 because prime(3)*prime(4) = 5*7 = 35, and 35 mod (2*3) = 5 and 35 mod (3*5) = 5 are prime.
MAPLE
N:= 100: # for a(1)..a(N)
P:= [seq(ithprime(i), i=1..N+1)]:
PP:= [seq(P[i]*P[i+1], i=1..N)]:
f:= proc(n) nops(select(t -> isprime(PP[n] mod PP[t]), [$1..n-1])) end proc:
map(f, [$1..N]);
PROG
(PARI) a(n) = sum(k=1, n-1, isprime(lift(Mod(prime(n)*prime(n+1), prime(k)*prime(k+1))))); \\ Michel Marcus, Jan 04 2021
CROSSREFS
KEYWORD
nonn
AUTHOR
J. M. Bergot and Robert Israel, Jan 03 2021
STATUS
approved