A285700 a(n) = Number of iterations x -> 2x-1 needed to get a nonprime number, when starting with x = n.

0, 3, 2, 0, 1, 0, 2, 0, 0, 0, 1, 0, 1, 0, 0, 0, 1, 0, 3, 0, 0, 0, 1, 0, 0, 0, 0, 0, 1, 0, 2, 0, 0, 0, 0, 0, 2, 0, 0, 0, 1, 0, 1, 0, 0, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 1, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 0, 1, 0, 1, 0, 0, 0, 0, 0, 3, 0, 0, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 2, 0, 0, 0, 1, 0, 1, 0, 0, 0, 1, 0, 1, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0

Offset: 1

Author

Antti Karttunen, Apr 26 2017

Keywords

• nonn

Links

• Antti Karttunen, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A000040, A005382 (positions of terms > 1), A010051, A181715, A285701.

Programs

Mathematica

Array[Length@ NestWhileList[2 # - 1 &, #, PrimeQ@ # &] - 1 &, 120] (* Michael De Vlieger, Apr 26 2017 *)

Python

from sympy import isprime
def a(n): return 0 if isprime(n) == 0 else 1 + a(2*n - 1) # Indranil Ghosh, Apr 26 2017

Scheme

(define (A285700 n) (if (zero? (A010051 n)) 0 (+ 1 (A285700 (+ n n -1)))))

Formula

If A010051(n) = 0 [when n is a nonprime], a(n) = 0, otherwise a(n) = 1 + a((2*n)-1).

a(A000040(n)) = A181715(n).