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A073477
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Least k such that 2^n = k^2-sigma(k)*phi(k).
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0
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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
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,1
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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
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FORMULA
| a(n) = min{k: A069249(k)=2^n}. - R. J. Mathar, Oct 01 2011
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PROG
| (PARI) a(n)=if(n<0, 0, x=1; while(abs(x^2-sigma(x)*eulerphi(x)-2^n)>0, x++)); x
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CROSSREFS
| Sequence in context: A016020 A119990 A036128 * A070351 A070350 A070349
Adjacent sequences: A073474 A073475 A073476 * A073478 A073479 A073480
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KEYWORD
| nonn
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AUTHOR
| Benoit Cloitre (benoit7848c(AT)orange.fr), Aug 26 2002
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EXTENSIONS
| Edited and extended by Klaus Brockhaus (klaus-brockhaus(AT)t-online.de), Aug 29 2002
a(26) and a(30) from R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Oct 01 2011
a(25), a(27)-a(29) and a(31)-a(34) from Donovan Johnson (donovan.johnson(AT)yahoo.com), Oct 02 2011
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