OFFSET
1,16
FORMULA
G.f.: (1/(1 - x))*Sum_{k>=1} x^(4^k). - Ilya Gutkovskiy, Jan 08 2017
PROG
(Magma) [ Ilog(4, n) : n in [1..150] ];
(PARI) a(n)=#digits(n, 4)-1 \\ Twice as fast as a(n)=for(i=0, n, (n>>=2)||return(i)); the naïve code a(n)=log(n)\log(4) works for standard realprecision=28 only up to n=4^47-5 and it is slower by another factor 2. - M. F. Hasler, Mar 11 2015
(PARI) A102572(n)=logint(n, 4) \\ M. F. Hasler, Nov 07 2019
CROSSREFS
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Dec 23 2006
STATUS
approved