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A053868
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Numbers n such that sum of divisors of n less than n is odd.
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4
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2, 3, 4, 5, 7, 8, 11, 13, 15, 16, 17, 18, 19, 21, 23, 27, 29, 31, 32, 33, 35, 36, 37, 39, 41, 43, 45, 47, 50, 51, 53, 55, 57, 59, 61, 63, 64, 65, 67, 69, 71, 72, 73, 75, 77, 79, 83, 85, 87, 89, 91, 93, 95, 97, 98, 99, 100, 101, 103, 105, 107, 109, 111, 113, 115, 117, 119
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,1
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COMMENTS
| Numbers which are twice squares, even squares or odd nonsquares.
In other words: numbers whose sum of proper divisors is odd. [From Omar E. Pol, Aug 14 2009]
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LINKS
| Reinhard Zumkeller, Table of n, a(n) for n = 1..10000
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PROG
| (Haskell)
import Data.List (findIndices)
a053868 n = a053868_list !! (n-1)
a053868_list = map (+ 1) $ findIndices odd $ map a001065 [1..]
-- Reinhard Zumkeller, Sep 15 2011
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CROSSREFS
| Cf. A001065, A028982.
Cf. A053869. [From Omar E. Pol, Aug 14 2009]
Sequence in context: A058586 A090467 A177202 * A081730 A162408 A162721
Adjacent sequences: A053865 A053866 A053867 * A053869 A053870 A053871
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KEYWORD
| nonn
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AUTHOR
| Henry Bottomley (se16(AT)btinternet.com), Mar 29 2000
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