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 A272060 Numbers n such that sigma((n-1)/2) + tau((n-1)/2) is prime. 1
 3, 5, 17, 257, 325, 1025, 65537, 82945, 202501, 250001, 2829125, 7496645, 10240001, 13675205, 16000001, 27060805, 48469445, 71402501, 133448705, 150062501, 156250001, 172186885, 182250001, 343064485, 354117125, 453519617, 467943425, 1235663105 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Numbers n such that A000203((n-1)/2) + A000005((n-1)/2) is a prime q. Corresponding values of primes q are in A055813. Prime terms are in A272061. The first 5 known Fermat primes from A019434 are in this sequence. LINKS FORMULA a(n) = 2*A064205(n) + 1. EXAMPLE sigma((17-1)/2) + tau((17-1)/2) = sigma(8) + tau(8) = 15 + 4 = 19; 19 is prime, so 17 is in the sequence. MATHEMATICA Select[Range[3, 10^7, 2], PrimeQ[DivisorSigma[1, #] + DivisorSigma[0, #]] &[(# - 1)/2] &] (* Michael De Vlieger, Apr 20 2016 *) PROG (MAGMA) [n: n in [3..1000000] | IsPrime(NumberOfDivisors((n-1) div 2) + SumOfDivisors((n-1) div 2)) and (n-1) mod 2 eq 0] (PARI) isok(n) = isprime(sigma((n-1)/2) + numdiv((n-1)/2)); lista(nn) = forstep (n=3, nn, 2, if (isok(n), print1(n, ", "))); \\ Michel Marcus, Apr 19 2016 (PARI) is(n)=my(f=factor(n\2)); n>2 && isprime(sigma(f)+numdiv(f)) && isprime(n) \\ Charles R Greathouse IV, Apr 29 2016 CROSSREFS Cf. A000005, A000203, A055813, A064205, A272061. Sequence in context: A056826 A278138 A273870 * A333873 A058910 A307843 Adjacent sequences:  A272057 A272058 A272059 * A272061 A272062 A272063 KEYWORD nonn AUTHOR Jaroslav Krizek, Apr 19 2016 STATUS approved

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Last modified August 9 17:13 EDT 2022. Contains 356026 sequences. (Running on oeis4.)