A181518 a(n) is the number for which 2^A181516(n)||(2*a(n))!
2, 4, 7, 11, 13, 16, 22, 25, 28, 35, 37, 38, 41, 47, 50, 52, 56, 59, 64, 67, 70, 76, 88, 93, 97, 98, 100, 117, 122, 133, 137, 140, 143, 148, 158, 171, 176, 179, 182, 186, 193, 196, 200, 203, 213, 218, 223, 227, 233, 234, 236, 242, 247, 248, 253, 262, 280, 290, 299
Offset: 1
Keywords
Programs
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Maple
isA011371 := proc(n) option remember; local k,a; for k from 0 do a := A011371(k) ; if a > n then return false; elif a = n then return true; end if; end do: end proc: A181516 := proc(n) option remember; local a; if n = 1 then 3; else for a from procname(n-1)+1 do if isprime(a) then if isA011371(a) then return a; end if; end if; end do; end if; end proc: A181518 := proc(n) for m from 1 do if A011371(m) = A181516(n) then return m/2 ; end if; end do: end proc: # R. J. Mathar, Nov 04 2010
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Mathematica
f[n_] := Sum[Floor[n/2^k], {k, 0, Floor[Log[2, n]]}] (* A005187 *) Select[Range[400], PrimeQ[f[#]] &] (* Clark Kimberling, Feb 13 2025 *)
Extensions
Corrected (88 inserted, 129 replaced by 179) and extended beyond 227 by R. J. Mathar, Nov 04 2010
Comments