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A066950 Numbers n such that phi(sigma(n)) + sigma(phi(n)) = 2n. 1
1, 11, 18, 75, 172, 372, 482, 732, 3276, 7965, 13068, 46476, 103488, 252240, 727956, 1795605, 6030528, 8769600, 15436092, 56747712, 224518995, 297780084, 317961000, 855489144, 2691137721 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

a(26) > 3*10^10. - Donovan Johnson, Mar 05 2014

LINKS

Table of n, a(n) for n=1..25.

EXAMPLE

phi(sigma(11)) + sigma(phi(11)) = phi(12) + sigma(10) = 4 + 18 = 2*11, so 11 is in the sequence.

MATHEMATICA

f[n_] := EulerPhi[DivisorSigma[1, n]]+DivisorSigma[1, EulerPhi[n]]; Select[Range[1, 10^5], f[ # ]==2*#&]

CROSSREFS

Cf. A000010, A000203.

Sequence in context: A168433 A302455 A303237 * A214495 A162011 A123248

Adjacent sequences:  A066947 A066948 A066949 * A066951 A066952 A066953

KEYWORD

more,nonn

AUTHOR

Joseph L. Pe, Jan 25 2002

EXTENSIONS

Edited by Dean Hickerson, Jan 26 2002

a(20)-a(25) from Donovan Johnson, Sep 16 2009

STATUS

approved

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Last modified June 14 10:04 EDT 2021. Contains 345025 sequences. (Running on oeis4.)