id:A067351 - OEIS Search Results

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Numbers n such that sigma(n)+phi(n) has exactly 2 distinct prime divisors.

+0
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; listen)

	OFFSET	`1,1`
	FORMULA	`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=2211; sigma(25)+phi(25)=31+20=51=3*17.`
	MATHEMATICA	`Select[ Range[ 1, 100 ], Length[ FactorInteger[ DivisorSigma[ 1, # ]+EulerPhi[ # ] ] ]==2& ]`
	CROSSREFS	`Cf. A000005, A000010, A000203, A001221, A065387, A067349, A067350.` `Sequence in context: A028747 A136806 A073803 this_sequence A067350 A028727 A028762` `Adjacent sequences: A067348 A067349 A067350 this_sequence A067352 A067353 A067354`
	KEYWORD	`nonn`
	AUTHOR	`Labos E. (labos(AT)ana.sote.hu), Jan 17 2002`
	EXTENSIONS	`Edited by Dean Hickerson (dean(AT)math.ucdavis.edu), Jan 20 2002`

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Last modified August 19 23:53 EDT 2008. Contains 142930 sequences.

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