A103161 GCD of reverse(2^n) and reverse(2^(n+1)), where reverse(k) = A004086(k), the decimal representation of k read backwards.

2, 4, 1, 1, 23, 1, 1, 1, 1, 4201, 2, 2, 1, 1, 1, 1, 1, 1, 1, 1, 2, 2, 1, 1, 1, 7, 1, 1, 1, 1, 4, 2, 1, 1, 1, 1, 1, 1, 1, 1, 2, 2, 19, 1, 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 4, 1, 1, 1, 1, 1, 1, 1, 1, 1, 34, 1, 1, 1, 7, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1

Offset: 1

Author

Labos Elemer, Jan 25 2005

Keywords

• base
• nonn

Examples

			n=10: GCD of backward written powers of 2 is GCD(4201, 8402) = 4201 = a(10).
		

Links

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

Crossrefs

Cf. A000079, A004094.

Programs

Mathematica

rd[x_] :=FromDigits[Reverse[IntegerDigits[x]]] Table[GCD[rd[2^w], rd[2^(w+1)]], {w, 1, 100}]
GCD[IntegerReverse[#[[1]]],IntegerReverse[#[[2]]]]&/@ Partition[ 2^Range[110],2,1] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Aug 24 2017 *)

PARI

rev(n) = subst(Polrev(digits(n)), 'x, 10); \\ These two functions from Charles R Greathouse IV, Oct 20 2014
A004094(n) = rev(2^n);
A103161(n) = gcd(A004094(n),A004094(1+n)); \\ Antti Karttunen, Dec 07 2017

Formula

a(n) = gcd(A004094(n), A004094(n+1)).