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 A266165 Numbers n such that n = 2* phi(sigma((n-1)/2)) + 1. 0
 3, 5, 17, 25, 257, 481, 1441, 13825, 65537, 285121, 1425601, 2280961, 2380801, 6690817, 7142401, 11404801, 29719873, 59439745, 100638721, 237758977, 4294967297, 7778073601, 8778792961 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Prime terms are in A260476. The first 5 known Fermat primes from A019434 are in the sequence. 100638721, 8778792961 and 184354652161 are also terms. LINKS Table of n, a(n) for n=1..23. EXAMPLE 17 = 2*phi(sigma((17-1)/2) + 1 = 2*phi(15) + 1 = 2*8 + 1, so 17 is in the sequence. MATHEMATICA Select[Range[10000], # == 2*EulerPhi[DivisorSigma[1, (# - 1)/2] ] + 1 &] (* G. C. Greubel, Dec 22 2015 *) PROG (Magma) [n: n in [3..10^7] | n eq 2*EulerPhi(SumOfDivisors((n-1) div 2)) + 1] (Perl) use ntheory ":all"; for (1..1e7) { say if 2*euler_phi(divisor_sum((\$_-1)>>1))+1 == \$_ } # Dana Jacobsen, Dec 27 2015 (PARI) is(n)=n%2 && n>2 && 2*eulerphi(sigma((n-1)/2)) + 1 == n \\ Charles R Greathouse IV, Apr 25 2016 CROSSREFS Cf. A000010, A062401, A260476. Sequence in context: A024867 A025111 A253204 * A281622 A256439 A256444 Adjacent sequences: A266162 A266163 A266164 * A266166 A266167 A266168 KEYWORD nonn,more AUTHOR Jaroslav Krizek, Dec 22 2015 EXTENSIONS More terms from Dana Jacobsen, Dec 27 2015 STATUS approved

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Last modified June 7 02:46 EDT 2023. Contains 363151 sequences. (Running on oeis4.)