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 A003597 Numbers of the form 3^i*11^j. 21
 1, 3, 9, 11, 27, 33, 81, 99, 121, 243, 297, 363, 729, 891, 1089, 1331, 2187, 2673, 3267, 3993, 6561, 8019, 9801, 11979, 14641, 19683, 24057, 29403, 35937, 43923, 59049, 72171, 88209, 107811, 131769, 161051, 177147, 216513, 264627, 323433 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS Reinhard Zumkeller, Table of n, a(n) for n = 1..10000 FORMULA The characteristic function of this sequence is given by Sum_{n >= 1} x^a(n) = Sum_{n >= 1} mu(33*n)*x^n/(1 - x^n), where mu(n) is the Möbius function A008683. Cf. with the formula of Hanna in A051037. - Peter Bala, Mar 18 2019 Sum_{n>=1} 1/a(n) = (3*11)/((3-1)*(11-1)) = 33/20. - Amiram Eldar, Sep 23 2020 a(n) ~ exp(sqrt(2*log(3)*log(11)*n)) / sqrt(33). - Vaclav Kotesovec, Sep 23 2020 MATHEMATICA fQ[n_]:=PowerMod[33, n, n] == 0; Select[Range[4*10^5], fQ] (* Vincenzo Librandi, Jun 27 2016 *) PROG (PARI) list(lim)=my(v=List(), N); for(n=0, log(lim)\log(11), N=11^n; while(N<=lim, listput(v, N); N*=3)); vecsort(Vec(v)) \\ Charles R Greathouse IV, Jun 28 2011 (Haskell) import Data.Set (singleton, deleteFindMin, insert) a003597 n = a003597_list !! (n-1) a003597_list = f \$ singleton (1, 0, 0) where    f s = y : f (insert (3 * y, i + 1, j) \$ insert (11 * y, i, j + 1) s')          where ((y, i, j), s') = deleteFindMin s -- Reinhard Zumkeller, May 15 2015 (MAGMA) [n: n in [1..4*10^5] | PrimeDivisors(n) subset [3, 11]]; // Vincenzo Librandi, Jun 27 2016 (GAP) Filtered([1..324000], n->PowerMod(33, n, n)=0); # Muniru A Asiru, Mar 19 2019 CROSSREFS Cf. A025612, A025616, A025621, A025625, A025629, A025632, A025634, A025635, A108761, A003596, A107988, A003598, A108698, A003599, A107788, A108687, A108779, A108090. Sequence in context: A191180 A191128 A057261 * A018705 A018381 A227969 Adjacent sequences:  A003594 A003595 A003596 * A003598 A003599 A003600 KEYWORD nonn,easy AUTHOR STATUS approved

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Last modified January 16 03:36 EST 2021. Contains 340195 sequences. (Running on oeis4.)