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A053224
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Numbers k for which sigma(k) < sigma(k+1).
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24
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1, 2, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31, 33, 35, 37, 39, 41, 43, 47, 49, 51, 53, 55, 57, 59, 61, 62, 63, 65, 67, 69, 71, 73, 74, 75, 77, 79, 81, 83, 85, 87, 89, 91, 93, 95, 97, 99, 101, 103, 107, 109, 111, 113, 115, 119, 121, 123, 125, 127, 129, 131
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OFFSET
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1,2
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COMMENTS
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The asymptotic density of this sequence is 1/2 (Erdős, 1936). - Amiram Eldar, Mar 19 2021
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LINKS
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MAPLE
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with(numtheory): seq( `if`(sigma(i) < sigma(i+1), i, [][]), i=1..134);
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MATHEMATICA
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Select[Range[150], DivisorSigma[1, #] < DivisorSigma[1, # + 1] &] (* Carl Najafi, Aug 16 2011 *)
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PROG
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(Haskell)
import Data.List (elemIndices)
a053224 n = a053224_list !! (n-1)
a053224_list = map (+ 1) $ elemIndices True $
zipWith (<) a000203_list $ tail a000203_list
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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