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A238985
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Zeroless 7-smooth numbers.
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8
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1, 2, 3, 4, 5, 6, 7, 8, 9, 12, 14, 15, 16, 18, 21, 24, 25, 27, 28, 32, 35, 36, 42, 45, 48, 49, 54, 56, 63, 64, 72, 75, 81, 84, 96, 98, 112, 125, 126, 128, 135, 144, 147, 162, 168, 175, 189, 192, 196, 216, 224, 225, 243, 245, 252, 256, 288, 294, 315, 324, 336
(list;
graph;
refs;
listen;
history;
text;
internal format)
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OFFSET
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1,2
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COMMENTS
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A001221(a(n)) <= 3 since 10 cannot divide a(n).
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LINKS
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FORMULA
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EXAMPLE
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a(12615) = 2^25 * 3^227 * 7^28.
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PROG
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(Haskell)
import Data.Set (singleton, deleteFindMin, fromList, union)
a238985 n = a238985_list !! (n-1)
a238985_list = filter ((== 1) . a168046) $ f $ singleton 1 where
f s = x : f (s' `union` fromList
(filter ((> 0) . (`mod` 10)) $ map (* x) [2, 3, 5, 7]))
where (x, s') = deleteFindMin s
(PARI) zf(n)=vecmin(digits(n))
list(lim)=my(v=List(), t, t1); for(e=0, log(lim+1)\log(7), t1=7^e; for(f=0, log(lim\t1+1)\log(3), t=t1*3^f; while(t<=lim, if(zf(t), listput(v, t)); t<<=1)); for(f=0, log(lim\t1+1)\log(5), t=t1*5^f; while(t<=lim, if(zf(t), listput(v, t)); t*=3))); Set(v)
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CROSSREFS
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KEYWORD
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nonn,base
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AUTHOR
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EXTENSIONS
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Keyword:fini and keyword:full removed by Jianing Song, Jan 28 2023 as finiteness is only conjectured.
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STATUS
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approved
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