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A068414 Numbers n such that sigma(n) = 3n - 2*phi(n). 4
1, 12, 56, 260, 992, 1976, 2156, 2754, 16256, 25232, 41072, 133984, 145888, 1100864, 1270208, 1439552, 2237888, 4729664, 67100672, 75398912, 171627376, 277060144, 473089984, 538178048, 558585344, 629225984, 1192258048, 1863840112, 2181070592, 4534854656 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

If 2^p-1 is prime(a Mersenne prime) and n=2^p*(2^p-1) then n is in the sequence because 3*n-2*phi(n)=3*2^p*(2^p-1)-2^p*(2^p-2) =2^p*(2^(p+1)-1)=sigma(2^p-1)*sigma(2^p)=sigma(2^p*(2^p-1))= sigma(n). - Farideh Firoozbakht, Dec 31 2005

LINKS

Table of n, a(n) for n=1..30.

PROG

(PARI) for(n=1, 500000, if(sigma(n)==3*n-2*eulerphi(n), print1(n, ", ")))

CROSSREFS

Cf. A068418, A069719, A069737.

Sequence in context: A275505 A009827 A068418 * A199316 A081756 A307741

Adjacent sequences:  A068411 A068412 A068413 * A068415 A068416 A068417

KEYWORD

nonn

AUTHOR

Benoit Cloitre, Mar 03 2002

EXTENSIONS

More terms (complete up to 50000000). - Rick L. Shepherd, Mar 28 2002

More terms from Labos Elemer, Apr 03 2002

a(24)-a(30) from Donovan Johnson, Feb 08 2012

STATUS

approved

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Last modified January 20 13:06 EST 2022. Contains 350472 sequences. (Running on oeis4.)