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 A256444 Numbers n such that sigma(n) = 2*(phi(n-1)+1). 2
 3, 5, 17, 26, 257, 65537, 10866583226 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Subsequence of A256439. Supersequence of Fermat primes (A019434). a(8) > 10^13. - Giovanni Resta, Jul 13 2015 LINKS EXAMPLE 17 is in the sequence because sigma(17) = 18 = 2*(phi(16-1)+1) = 2*9. MATHEMATICA Select[Range@ 100000, DivisorSigma[1, #] == 2 (EulerPhi[# - 1] + 1) &] (* Michael De Vlieger, Mar 31 2015 *) PROG (MAGMA) Set(Sort([n: n in [2..1000000] | SumOfDivisors(n) / (EulerPhi(n-1) + 1) eq 2 ])) (PARI) first(m)={ my(v=vector(m), i, r); r=0; for(i=1, m, until(sigma(r)===2*(eulerphi(r-1)+1), r++); v[i]=r; print1(r, ", "); ); v; } Anders HellstrÃ¶m, Jul 29 2015 CROSSREFS Cf. A000010, A000203, A256439. Sequence in context: A266165 A281622 A256439 * A032619 A193066 A193070 Adjacent sequences:  A256441 A256442 A256443 * A256445 A256446 A256447 KEYWORD nonn,more AUTHOR Jaroslav Krizek, Mar 31 2015 EXTENSIONS a(7) from Giovanni Resta, Jul 13 2015 STATUS approved

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Last modified April 21 12:37 EDT 2021. Contains 343150 sequences. (Running on oeis4.)