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A067351 Numbers n such that sigma(n) + phi(n) has exactly 2 distinct prime divisors. 3
3, 5, 6, 7, 10, 11, 13, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 33, 35, 37, 39, 40, 41, 42, 43, 44, 46, 47, 49, 51, 52, 53, 55, 56, 57, 58, 59, 60, 61, 64, 66, 67, 68, 71, 72, 73, 75, 76, 78, 79, 80, 81, 82, 83, 84, 85, 87, 89, 91, 92, 93, 95, 96, 97 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

G. C. Greubel, Table of n, a(n) for n = 1..1000

FORMULA

a(n) = A001221(A000010(n) + A000203(n)) = A001221(A065387(n)) = 2.

EXAMPLE

Includes all odd primes and some composites; e.g., 21 and 25, since sigma(21) + phi(21) = 32 + 12 = 44 = 2*2*11; sigma(25) + phi(25) = 31 + 20 = 51 = 3*17.

MATHEMATICA

Select[ Range[ 1, 100 ], Length[ FactorInteger[ DivisorSigma[ 1, # ]+EulerPhi[ # ] ] ]==2& ]

Select[Range[500], PrimeNu[EulerPhi[#] + DivisorSigma[1, #]] == 2 &] (* G. C. Greubel, May 08 2017 *)

CROSSREFS

Cf. A000005, A000010, A000203, A001221, A065387, A067349, A067350.

Sequence in context: A292763 A176175 A157201 * A067350 A176651 A028727

Adjacent sequences:  A067348 A067349 A067350 * A067352 A067353 A067354

KEYWORD

nonn

AUTHOR

Labos Elemer, Jan 17 2002

EXTENSIONS

Edited by Dean Hickerson, Jan 20 2002

STATUS

approved

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Last modified October 22 09:20 EDT 2017. Contains 293761 sequences.