login
This site is supported by donations to The OEIS Foundation.

 

Logo

Many excellent designs for a new banner were submitted. We will use the best of them in rotation.

Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A067351 Numbers n such that sigma(n)+phi(n) has exactly 2 distinct prime divisors. 2
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

Table of n, a(n) for n=1..70.

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=2*2*11; 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: A182851 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 (dean.hickerson(AT)yahoo.com), Jan 20 2002

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Transforms | Superseeker | Recent | More pages
The OEIS Community | Maintained by The OEIS Foundation Inc.

Content is available under The OEIS End-User License Agreement .

Last modified April 23 11:09 EDT 2014. Contains 240919 sequences.