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 A109925 Number of primes of the form n - 2^k. 14
 0, 0, 1, 2, 1, 2, 2, 1, 2, 1, 2, 1, 2, 1, 3, 0, 1, 2, 3, 1, 4, 0, 2, 1, 2, 0, 3, 0, 1, 1, 2, 1, 3, 1, 3, 0, 2, 1, 4, 0, 1, 1, 2, 1, 5, 0, 2, 1, 3, 0, 3, 0, 1, 1, 3, 0, 2, 0, 1, 1, 3, 1, 4, 0, 1, 1, 2, 1, 5, 0, 2, 1, 2, 1, 6, 0, 3, 0, 2, 1, 3, 0, 3, 1, 2, 0, 4, 0, 1, 1, 3, 0, 3, 0, 2, 0, 1, 1, 3, 0, 2, 1, 2, 1, 6 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,4 COMMENTS Erdos conjectures that the numbers in A039669 are the only n for which n-2^r is prime for all 2^r0; A118952(n)<=a(n); A078687(n)=a(A000040(n)). - Reinhard Zumkeller, May 07 2006 G.f.: ( Sum_{i>=0} x^(2^i) ) * ( Sum_{j>=1} x^prime(j) ). - Ilya Gutkovskiy, Feb 10 2022 EXAMPLE a(21) = 4, 21-2 =19, 21-4 = 17, 21-8 = 13, 21-16 = 5, four primes. 127 is the smallest odd number > 1 such that a(n) = 0: A006285(2) = 127. - Reinhard Zumkeller, May 27 2015 MAPLE A109925 := proc(n)     a := 0 ;     for k from 0 do         if n-2^k < 2 then             return a ;         elif isprime(n-2^k) then             a := a+1 ;         end if;     end do: end proc: seq(A109925(n), n=1..80) ; # R. J. Mathar, Mar 07 2022 MATHEMATICA Table[cnt=0; r=1; While[r

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Last modified May 16 05:56 EDT 2022. Contains 353693 sequences. (Running on oeis4.)