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A025620
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Numbers of the form 4^i * 9^j, with i, j >= 0.
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4
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1, 4, 9, 16, 36, 64, 81, 144, 256, 324, 576, 729, 1024, 1296, 2304, 2916, 4096, 5184, 6561, 9216, 11664, 16384, 20736, 26244, 36864, 46656, 59049, 65536, 82944, 104976, 147456, 186624, 236196, 262144, 331776, 419904, 531441, 589824, 746496, 944784
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OFFSET
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1,2
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COMMENTS
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LINKS
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FORMULA
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Sum_{n>=1} 1/a(n) = (4*9)/((4-1)*(9-1)) = 3/2. - Amiram Eldar, Sep 24 2020
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MATHEMATICA
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n = 10^6; Flatten[Table[4^i*9^j, {i, 0, Log[4, n]}, {j, 0, Log[9, n/4^i]}]] // Sort (* Amiram Eldar, Sep 24 2020 *)
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PROG
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(Haskell)
import Data.Set (singleton, deleteFindMin, insert)
a025620 n = a025620_list !! (n-1)
a025620_list = f $ singleton 1 where
f s = y : f (insert (4 * y) $ insert (9 * y) s')
where (y, s') = deleteFindMin s
(PARI) list(lim)=my(v=List(), N); for(n=0, logint(lim\=1, 9), N=9^n; while(N<=lim, listput(v, N); N<<=2)); Set(v) \\ Charles R Greathouse IV, Jan 10 2018
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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STATUS
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approved
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