A046268 - OEIS

A046268

Largest prime substring in 2^n (or 0 if none exist).

0, 2, 0, 0, 0, 3, 0, 2, 5, 5, 2, 2, 409, 19, 163, 7, 6553, 131, 2621, 5, 857, 971, 419, 83, 1677721, 5443, 71, 1342177, 43, 709, 107, 83, 4294967, 89, 171798691, 3597383, 6871947673, 3743895347, 779069, 54975581, 511627, 99023, 4398046511, 79609, 18604441, 883 (list; graph; refs; listen; history; text; internal format)

	OFFSET	`0,2`
	LINKS	`Michael S. Branicky, Table of n, a(n) for n = 0..2000`
	EXAMPLE	`2^37 = 1{3743895347}2, so a(37) = 3743895347.`
	PROG	`(Python)` `from sympy import isprime` `def a(n):` `s = str(2**n)` `ss = (int(s[i:j]) for i in range(len(s)) for j in range(i+1, len(s)+1))` `return max((k for k in ss if isprime(k)), default=0)` `print([a(n) for n in range(46)]) # Michael S. Branicky, Sep 20 2022`
	CROSSREFS	`Cf. A046260.` `Sequence in context: A348341 A330936 A284687 * A202425 A292374 A292376` `Adjacent sequences: A046265 A046266 A046267 * A046269 A046270 A046271`
	KEYWORD	`nonn`
	AUTHOR	`Patrick De Geest, Jun 15 1998`
	EXTENSIONS	`a(44) and beyond from Michael S. Branicky, Sep 20 2022`
	STATUS	`approved`