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Search: id:A051488
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| A051488 |
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Numbers n such that phi(n) < phi(n - phi(n)). |
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+0
2
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| 30, 60, 66, 120, 132, 138, 174, 210, 240, 246, 264, 276, 318, 330, 348, 420, 480, 492, 498, 510, 528, 534, 552, 630, 636, 660, 678, 690, 696, 786, 840, 870, 910, 960, 984, 996, 1020, 1038, 1056, 1068, 1074, 1104, 1122, 1146, 1260, 1272, 1320, 1330, 1356
(list; graph; listen)
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OFFSET
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1,1
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COMMENT
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If p is a Sophie Germain prime greater than 3 and n is a natural number then 2^n*3*p is in the sequence. That is because if m= 2^n*3*p then phi(m)=2^n*(p-1) and phi(m - phi(m))=phi(2^n*3*p - 2^n*(p-1))=phi(2^n*(2p+1))=2^n*p so phi(m)<phi(m-phi(m)) and m is in the sequence. - Farideh Firoozbakht (mymontain(AT)yahoo.com), Jun 19 2005
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REFERENCES
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R. K. Guy, Unsolved Problems in Number Theory B42.
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LINKS
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T. D. Noe, Table of n, a(n) for n=1..1000
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MATHEMATICA
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Select[Range[1360], EulerPhi[ # ] < EulerPhi[ # - EulerPhi[ # ]] &] (Firoozbakht)
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CROSSREFS
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Cf. A051487, A005384.
Sequence in context: A004962 A121960 A040870 this_sequence A051283 A066031 A071140
Adjacent sequences: A051485 A051486 A051487 this_sequence A051489 A051490 A051491
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KEYWORD
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nonn,nice,easy
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AUTHOR
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R. K. Guy (rkg(AT)cpsc.ucalgary.ca)
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EXTENSIONS
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More terms from James A. Sellers (sellersj(AT)math.psu.edu)
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