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A225495
Numbers having only weak prime factors, cf. A051635.
5
1, 3, 7, 9, 13, 19, 21, 23, 27, 31, 39, 43, 47, 49, 57, 61, 63, 69, 73, 81, 83, 89, 91, 93, 103, 109, 113, 117, 129, 131, 133, 139, 141, 147, 151, 161, 167, 169, 171, 181, 183, 189, 193, 199, 207, 217, 219, 229, 233, 241, 243, 247, 249, 267, 271, 273, 279
OFFSET
1,2
LINKS
FORMULA
Multiplicative closure of A051635.
EXAMPLE
a(10) = 31 = A051635(6);
a(11) = 39 = 3 * 13 = A051635(1) * A051635(3);
a(12) = 43 = A051635(7);
a(13) = 47 = A051635(8);
a(14) = 49 = 7^2 = A051635(2)^2;
a(15) = 57 = 3 * 19 = A051635(1) * A051635(4).
PROG
(Haskell)
import Data.Set (singleton, fromList, union, deleteFindMin)
a225495 n = a225495_list !! (n-1)
a225495_list = 1 : h (singleton p) ps [p] where
(p:ps) = a051635_list
h s xs'@(x:xs) ys
| m > x = h (s `union` (fromList $ map (* x) (1 : ys))) xs ys
| otherwise = m : h (s' `union` (fromList $ map (* m) ys')) xs' ys'
where ys' = m : ys; (m, s') = deleteFindMin s
CROSSREFS
Cf. A225493 (strong), A225494 (balanced), A225496 (non-balanced).
Sequence in context: A023490 A032375 A089556 * A111223 A266836 A066207
KEYWORD
nonn
AUTHOR
Reinhard Zumkeller, May 09 2013
STATUS
approved