OFFSET
1,1
COMMENTS
If 2^p-1 is a prime (a Mersenne prime) greater than 3 then 3*2^p*(2^p-1) is in the sequence (the proof is easy). The sequence A110075 gives such terms of this sequence. - Farideh Firoozbakht, Jul 27 2005
If 2^p-1 is a prime (a Mersenne prime) not equal to 7 then 7*2^(p+1)*(2^p-1) is in the sequence (the proof is easy). - Farideh Firoozbakht, Aug 18 2013
Theorem: If 2^p-1 and 2^q-1 are two distinct Mersenne primes then 2^(p+q-2)*(2^p-1)*(2^q-1) is in the sequence (the proof is easy). The two preceding remarks are the special cases q = 2 and q = 3. - Farideh Firoozbakht, Dec 21 2014
MATHEMATICA
a068420[n_] := Select[Range[n], DivisorSigma[1, #] == 4 (# - EulerPhi[#]) &]; a068420[10^6] (* Michael De Vlieger, Dec 21 2014 *)
PROG
(PARI) for(n=1, 100000000, if(sigma(n)==4*(n-eulerphi(n)), print1(n, ", ")))
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Benoit Cloitre, Mar 02 2002
EXTENSIONS
More terms from Rick L. Shepherd, Apr 03 2002
a(29)-a(32) from Donovan Johnson, Jun 30 2012
STATUS
approved