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 A067260 Numbers k such that sigma(k+1) = 2*phi(k). 3
 13, 43, 109, 151, 589, 883, 2143, 2725, 4825, 4921, 9541, 13189, 21637, 22249, 22489, 29971, 30229, 33787, 36247, 72541, 73513, 83287, 94489, 109213, 113269, 117367, 189103, 190489, 198457, 216529, 247597, 277447, 297307, 320137, 353821, 357751, 376753, 391543 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS If p=2^n+3 and both numbers p & q=(1/2)*(p^2-3p-2) are primes then q is in the sequence, because sigma(q+1)=sigma((1/2)*(p-3)*p)= sigma(2^(n-1)*p)=(2^n-1)*(p+1)=(p-4)*(p+1)=p^2-3p-4=2q-2=2*phi(q). 13, 43, 151, 2143 & 34360131583 are such terms corresponding to n = 2, 3, 4, 6 & 18. - Farideh Firoozbakht, Feb 16 2008 LINKS Amiram Eldar, Table of n, a(n) for n = 1..1000 MATHEMATICA Do[If[DivisorSigma[1, n+1]==2*EulerPhi@n, Print[n]], {n, 200000}] (* Farideh Firoozbakht, Feb 16 2008 *) CROSSREFS Cf. A000010, A000203, A067261, A067262, A067263, A135241. Sequence in context: A106734 A066465 A023262 * A135241 A225774 A268256 Adjacent sequences: A067257 A067258 A067259 * A067261 A067262 A067263 KEYWORD nonn AUTHOR Benoit Cloitre, Feb 21 2002 EXTENSIONS More terms from Amiram Eldar, Apr 24 2022 STATUS approved

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Last modified August 11 05:51 EDT 2024. Contains 375059 sequences. (Running on oeis4.)