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A002181
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Least number k such that phi(k) = m, where m runs through the values (A002202) taken by phi.
(Formerly M2421 N0957)
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19
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1, 3, 5, 7, 15, 11, 13, 17, 19, 25, 23, 35, 29, 31, 51, 37, 41, 43, 69, 47, 65, 53, 81, 87, 59, 61, 85, 67, 71, 73, 79, 123, 83, 129, 89, 141, 97, 101, 103, 159, 107, 109, 121, 113, 177, 143, 127, 255, 131, 161, 137, 139, 213, 185, 149, 151, 157, 187, 163, 249, 167, 203, 173
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OFFSET
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1,2
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COMMENTS
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Inverse of Euler totient function.
According to Guy, the first even term is for 2m = 16842752 = 257*2^16. If there are only five Fermat primes, then terms will be even for 2m = 2^r for all r > 31. This was discussed in problem E3361. - T. D. Noe, Aug 14 2008
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REFERENCES
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J. W. L. Glaisher, Number-Divisor Tables. British Assoc. Math. Tables, Vol. 8, Camb. Univ. Press, 1940, p. 64.
R. K. Guy, Unsolved problems in number theory, B39.
N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
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William P. Wardlaw, L. L. Foster and R. J. Simpson, Problem E3361, Amer. Math. Monthly, Vol. 98, No. 5 (May, 1991), 443-444.
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FORMULA
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MATHEMATICA
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With[{ep=EulerPhi[Range[1000]]}, Flatten[Table[Position[ep, n, {1}, 1], {n, 200}]]] (* Harvey P. Dale, Apr 10 2015 *)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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Offset and initial term corrected Oct 07 2007
Revised definition from T. D. Noe, Aug 14 2008
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STATUS
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approved
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