A073477 Least k such that 2^n = k^2-sigma(k)*phi(k).

2, 4, 8, 16, 32, 12, 20, 256, 44, 1024, 2048, 4096, 8192, 16384, 992, 65536, 724, 262144, 2080, 1048576, 16256, 4194304, 8388608, 16777216, 33554432, 67108864, 48832, 268435456, 536870912, 1073741824, 471808, 4294967296, 8589934592, 17179869184, 34359738368

Offset

0,1

Comments

Sequence is always defined since for s=2^(n+1), 2^n = s^2-sigma(s)*phi(s). - R. J. Mathar, Oct 01 2011

a(38) = 67100672. a(50) = 17179738112. a(56) <= 274877382656. - Donovan Johnson, Oct 02 2011

Links

Table of n, a(n) for n=0..34.

Formula

a(n) = min{k: A069249(k)=2^n}. - R. J. Mathar, Oct 01 2011

Prog

(PARI) a(n)=if(n<0, 0, x=1; while(abs(x^2-sigma(x)*eulerphi(x)-2^n)>0, x++)); x

Crossrefs

Sequence in context: A328753 A119990 A036128 * A220105 A070351 A070350

Adjacent sequences: A073474 A073475 A073476 * A073478 A073479 A073480

Keyword

nonn

Author

Benoit Cloitre, Aug 26 2002

Extensions

Edited and extended by Klaus Brockhaus, Aug 29 2002

a(26) and a(30) from R. J. Mathar, Oct 01 2011

a(25), a(27)-a(29) and a(31)-a(34) from Donovan Johnson, Oct 02 2011

Status

approved