A131539 Exponent of least power of 2 having exactly n consecutive 5's in its decimal representation.

0, 8, 16, 76, 41, 1162, 973, 6838, 25265, 81782, 456686, 279270, 1727606, 6030753, 23157026, 106892455

Offset: 0

Author

Shyam Sunder Gupta, Aug 26 2007

Keywords

• more
• nonn
• base

Examples

			a(3)=76 because 2^76 (i.e., 75557863725914323419136) is the smallest power of 2 to contain a run of 3 consecutive fives in its decimal form.
		

Links

• Popular Computing (Calabasas, CA), Two Tables, Vol. 1, (No. 9, Dec 1973), page PC9-16.

Programs

Mathematica

a = ""; Do[ a = StringJoin[a, "5"]; b = StringJoin[a, "5"]; k = 1; While[ StringPosition[ ToString[2^k], a] == {} || StringPosition[ ToString[2^k], b] != {}, k++ ]; Print[k], {n, 1, 10} ]

Extensions

2 more terms from Sean A. Irvine, Jul 19 2010

a(13)-a(14) from Lars Blomberg, Jan 24 2013

a(0)=0 prepended by and a(15) from Paul Geneau de Lamarlière, Jul 19 2024