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 A068420 Numbers n such that sigma(n) = 4*(n-phi(n)). 2
 3, 99, 168, 780, 1836, 2976, 5928, 6201, 6468, 13888, 48768, 75696, 123216, 227584, 285948, 401952, 437664, 1003000, 2058732, 3302592, 3810624, 4031488, 4258496, 4318656, 6713664, 14188992, 32021613, 93298284, 201302016, 226196736, 381144320, 514882128 (list; graph; refs; listen; history; text; internal format)
 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 LINKS 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 Cf. A000668, A110075. Sequence in context: A246537 A057014 A167582 * A276188 A180350 A293952 Adjacent sequences:  A068417 A068418 A068419 * A068421 A068422 A068423 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

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Last modified October 14 16:48 EDT 2019. Contains 328022 sequences. (Running on oeis4.)