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A051488 Numbers n such that phi(n) < phi(n - phi(n)). 2
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; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

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, Jun 19 2005

REFERENCES

R. K. Guy, Unsolved Problems in Number Theory B42.

LINKS

T. D. Noe, Table of n, a(n) for n=1..1000

MATHEMATICA

Select[Range[1360], EulerPhi[ # ] < EulerPhi[ # - EulerPhi[ # ]] &] (Firoozbakht)

PROG

(Haskell)

a051488 n = a051488_list !! (n-1)

a051488_list = [x | x <- [2..], let t = a000010 x, t < a000010 (x - t)]

-- Reinhard Zumkeller, Apr 12 2014

CROSSREFS

Cf. A051487, A005384.

Cf. A000010.

Sequence in context: A040870 A280011 A175785 * A305613 A267967 A051283

Adjacent sequences:  A051485 A051486 A051487 * A051489 A051490 A051491

KEYWORD

nonn,nice,easy

AUTHOR

R. K. Guy

EXTENSIONS

More terms from James A. Sellers

STATUS

approved

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Last modified November 18 13:22 EST 2018. Contains 317306 sequences. (Running on oeis4.)