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A238985 Zeroless 7-smooth numbers. 4
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)
OFFSET

1,2

COMMENTS

A001221(a(n)) <= 3 since 10 cannot divide a(n).

LINKS

Charles R Greathouse IV, Table of n, a(n) for n = 1..10000

FORMULA

A086299(a(n)) * A168046(a(n)) = 1.

EXAMPLE

a(12615) = 2^25 * 3^227 * 7^28.

PROG

(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)

CROSSREFS

Cf. A168046, intersection of A002473 and A052382.

Cf. A059405 (subsequence).

Sequence in context: A087087 A050742 A290387 * A226900 A111228 A283248

Adjacent sequences:  A238982 A238983 A238984 * A238986 A238987 A238988

KEYWORD

nonn,base

AUTHOR

Charles R Greathouse IV and Reinhard Zumkeller, Mar 07 2014

STATUS

approved

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Last modified March 19 04:17 EDT 2019. Contains 321311 sequences. (Running on oeis4.)