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 A247954 a(n) = sigma(sigma(2n-1)). 8
 1, 7, 12, 15, 14, 28, 24, 60, 39, 42, 63, 60, 32, 90, 72, 63, 124, 124, 60, 120, 96, 84, 168, 124, 80, 195, 120, 195, 186, 168, 96, 210, 224, 126, 252, 195, 114, 224, 252, 186, 133, 224, 280, 360, 234, 248, 255, 360, 171, 392, 216, 210, 508, 280, 216, 300 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS See A247821 - numbers n such that sigma(sigma(2n-1)) is a prime p. LINKS Indranil Ghosh, Table of n, a(n) for n = 1..10000 FORMULA a(n) = A000203(A000203(2n-1)) = A000203(A008438(n-1)) = A051027(2n-1). EXAMPLE For n=2; a(2) = sigma(sigma(2*2-1)) = sigma(sigma(3)) = sigma(4) = 7. MAPLE with(numtheory): A247954:=n->sigma(sigma(2*n-1)): seq(A247954(n), n=1..50); # Wesley Ivan Hurt, Oct 01 2014 MATHEMATICA Table[DivisorSigma[1, DivisorSigma[1, 2 n - 1]], {n, 50}] (* Wesley Ivan Hurt, Oct 01 2014 *) PROG (MAGMA) [SumOfDivisors(SumOfDivisors(2*n-1)): n in [1..1000]] (PARI) vector(100, n, sigma(sigma(2*n-1))) \\ Derek Orr, Sep 29 2014 CROSSREFS Cf. A000203, A051027, A008438, A247820. Sequence in context: A287562 A073255 A162194 * A145310 A177988 A072834 Adjacent sequences:  A247951 A247952 A247953 * A247955 A247956 A247957 KEYWORD nonn,easy AUTHOR Jaroslav Krizek, Sep 28 2014 STATUS approved

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Last modified May 6 11:12 EDT 2021. Contains 343581 sequences. (Running on oeis4.)