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A073477
Least k such that 2^n = k^2-sigma(k)*phi(k).
0
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
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
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