OFFSET
1,4
COMMENTS
EXAMPLE
47 -> 23 -> 11 -> 5 -> 2 (prime(15) -> prime(9) -> prime(5) -> prime(3) -> prime(1)), hence a(1) = 0, a(3) = 1, a(5) = 2, a(9) = 3, a(15) = 4.
7 -> 3 -> 1 (prime(4) -> prime(2) -> 1), hence a(2) = 1, a(4) = 2.
59 -> 29 -> 14 (prime(17) -> prime(10) -> 14), hence a(10) = 1, a(17) = 2.
MATHEMATICA
a[n_] := -1 + Length @ NestWhileList[(# - 1)/2 &, Prime[n], OddQ[#] && PrimeQ[#] &]; Array[a, 90] (* Amiram Eldar, Nov 27 2021 *)
PROG
(PARI) a(n) = my(p=prime(n), k=0); while(isprime(m = (p+1)>>k - 1) && m != 2, k++); k
CROSSREFS
KEYWORD
nonn
AUTHOR
Jianing Song, Nov 24 2021
STATUS
approved