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A108090
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Numbers of the form (11^i)*(13^j).
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20
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1, 11, 13, 121, 143, 169, 1331, 1573, 1859, 2197, 14641, 17303, 20449, 24167, 28561, 161051, 190333, 224939, 265837, 314171, 371293, 1771561, 2093663, 2474329, 2924207, 3455881, 4084223, 4826809, 19487171, 23030293, 27217619
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OFFSET
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1,2
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LINKS
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FORMULA
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Sum_{n>=1} 1/a(n) = (11*13)/((11-1)*(13-1)) = 143/120. - Amiram Eldar, Sep 23 2020
a(n) ~ exp(sqrt(2*log(11)*log(13)*n)) / sqrt(143). - Vaclav Kotesovec, Sep 23 2020
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MATHEMATICA
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mx = 3*10^7; Sort@ Flatten@ Table[ 11^i*13^j, {i, 0, Log[11, mx]}, {j, 0, Log[13, mx/11^i]}] (* Robert G. Wilson v, Aug 17 2012 *)
fQ[n_]:=PowerMod[143, n, n] == 0; Select[Range[2 10^7], fQ] (* Vincenzo Librandi, Jun 27 2016 *)
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PROG
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(Haskell)
import Data.Set (singleton, deleteFindMin, insert)
a108090 n = a108090_list !! (n-1)
a108090_list = f $ singleton (1, 0, 0) where
f s = y : f (insert (11 * y, i + 1, j) $ insert (13 * y, i, j + 1) s')
where ((y, i, j), s') = deleteFindMin s
(Magma) [n: n in [1..10^7] | PrimeDivisors(n) subset [11, 13]]; // Vincenzo Librandi, Jun 27 2016
(PARI) list(lim)=my(v=List(), t); for(j=0, logint(lim\=1, 13), t=13^j; while(t<=lim, listput(v, t); t*=11)); Set(v) \\ Charles R Greathouse IV, Aug 29 2016
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CROSSREFS
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Cf. A003586, A003592, A003593, A003591, A003594, A003595, A003596, A003597, A003598, A003599, A107326, A107364, A107466, A108056.
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KEYWORD
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nonn,easy
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AUTHOR
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Douglas Winston (douglas.winston(AT)srupc.com), Jun 03 2005
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STATUS
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approved
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