A025742 a(n) is the index of 10^n within sequence of numbers of form 4^i*10^j.

1, 3, 7, 12, 19, 28, 38, 50, 64, 79, 96, 115, 135, 157, 181, 206, 233, 262, 292, 324, 358, 393, 430, 469, 509, 551, 595, 640, 687, 736, 786, 838, 892, 947, 1004, 1063, 1123, 1185, 1249, 1314, 1381, 1450, 1520, 1592, 1666, 1741, 1818, 1897, 1977, 2059, 2143, 2228

Offset: 0

Author

David W. Wilson

Keywords

• nonn
• easy

Comments

Indices m for which A025649(m) is 0. - Michel Marcus, Jan 10 2018

Examples

			The index of 10^0 in A025621 is 1, so a(0)=1.
The index of 10^1 in A025621 is 3, so a(1)=3.
The index of 10^2 in A025621 is 7, so a(2)=7.
		

Links

• Charles R Greathouse IV, Table of n, a(n) for n = 0..10000

Crossrefs

Cf. A025621, A025649, A025686.

Programs

PARI

a(n)=my(N=1); n+1+sum(i=1,n, logint(N*=10,4)); \\ Charles R Greathouse IV, Jan 10 2018

PARI

first(n)=my(s,N=1/10); vector(n+1,i, s+=logint(N*=10,4)+1) \\ Charles R Greathouse IV, Jan 10 2018

Extensions

Offset changed by Michel Marcus, Jan 10 2018