This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A285700 #11 Apr 26 2017 21:48:03 %S A285700 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, %T A285700 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, %U A285700 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 %N A285700 a(n) = Number of iterations x -> 2x-1 needed to get a nonprime number, when starting with x = n. %H A285700 Antti Karttunen, <a href="/A285700/b285700.txt">Table of n, a(n) for n = 1..10000</a> %F A285700 If A010051(n) = 0 [when n is a nonprime], a(n) = 0, otherwise a(n) = 1 + a((2*n)-1). %F A285700 a(A000040(n)) = A181715(n). %t A285700 Array[Length@ NestWhileList[2 # - 1 &, #, PrimeQ@ # &] - 1 &, 120] (* _Michael De Vlieger_, Apr 26 2017 *) %o A285700 (Scheme) (define (A285700 n) (if (zero? (A010051 n)) 0 (+ 1 (A285700 (+ n n -1))))) %o A285700 (Python) %o A285700 from sympy import isprime %o A285700 def a(n): return 0 if isprime(n) == 0 else 1 + a(2*n - 1) # _Indranil Ghosh_, Apr 26 2017 %Y A285700 Cf. A000040, A005382 (positions of terms > 1), A010051, A181715, A285701. %K A285700 nonn %O A285700 1,2 %A A285700 _Antti Karttunen_, Apr 26 2017