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A175853
(2*n-1)-perfect numbers.
4
20, 128768, 33501184
OFFSET
1,1
COMMENTS
For the definition see A175837.
Considering only terms of the form 2^k * p with p prime results in a linear equation for p for a given k. Solving quickly generates other terms of the sequence: 137433972736, 2199001235456, 649037107316852462774394019643392, and a 157-digit term associated with k=260. - D. S. McNeil, Dec 08 2010
FORMULA
A033880(a(n))=A000005(a(n))/2-1.
EXAMPLE
Proper divisors of 20 are: 1,2,4,5,10. Since 2*20-1=(2*1-1)+(2*2-1)+(2*4-1)+(2*5-1)+(2*10-1)=39, then 20 is in the sequence.
CROSSREFS
Cf. A175837 (abundant version), A175522, A033880, A000005.
Sequence in context: A165344 A348646 A222745 * A340167 A013766 A078354
KEYWORD
nonn,hard,more,bref
AUTHOR
Vladimir Shevelev, Dec 05 2010
EXTENSIONS
a(3) from D. S. McNeil, Dec 08 2010
STATUS
approved