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 A157242 Number of ways to write the n-th positive odd integer in the form p+2^x+11*2^y with p a prime congruent to 5 mod 6 and x,y positive integers. 2
 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 0, 2, 1, 0, 2, 2, 0, 2, 2, 1, 3, 2, 2, 3, 1, 2, 4, 2, 2, 5, 1, 2, 5, 2, 2, 4, 2, 2, 3, 2, 3, 4, 4, 3, 6, 2, 3, 6, 5, 1, 7, 4, 2, 6 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,18 COMMENTS On Feb. 24, 2009, Zhi-Wei Sun conjectured that a(n)=0 if and only if n<15 or n=17, 20, 23, 86, 124; in other words, except for 33, 39, 45, 171 and 247, any odd integer greater than 28 can be written as the sum of a prime p=5 (mod 6), a positive power of 2 and eleven times a positive power of 2. Sun verified the conjecture for odd integers below 5*10^7. Knowing the conjecture from Sun, Qing-Hu Hou and D. S. McNeil have continued the verification for odd integers below 1.5*10^8 and 10^12 respectively, and they have found no counterexample. Compare the conjecture with Crocker's result that there are infinitely many positive odd integers not of the form p+2^x+2^y with p an odd prime and x,y positive integers. REFERENCES R. Crocker, On a sum of a prime and two powers of two, Pacific J. Math. 36(1971), 103-107. Z. W. Sun and M. H. Le, Integers not of the form c(2^a+2^b)+p^{alpha}, Acta Arith. 99(2001), 183-190. LINKS Zhi-Wei Sun, Table of n, a(n) for n=1..200000 Zhi-Wei Sun, A webpage: Mixed Sums of Primes and Other Terms, 2009. Zhi-Wei Sun, A project for the form p+2^x+k*2^y with k=3,5,...,61 Zhi-Wei Sun, A promising conjecture: n=p+F_s+F_t Z. W. Sun, Mixed sums of primes and other terms, preprint, 2009. arXiv:0901.3075 FORMULA a(n)=|{: p+2^x+11*2^y=2n-1 with p a prime congruent to 5 mod 6 and x,y positive integers}| EXAMPLE For n=18 the a(18)=2 solutions are 2*18-1=5+2^3+2*11=11+2+2*11. MATHEMATICA PQ[x_]:=x>1&&Mod[x, 6]==5&&PrimeQ[x] RN[n_]:=Sum[If[PQ[2n-1-11*2^x-2^y], 1, 0], {x, 1, Log[2, (2n-1)/11]}, {y, 1, Log[2, Max[2, 2n-1-11*2^x]]}] Do[Print[n, " ", RN[n]], {n, 1, 200000}] CROSSREFS A000040, A000079, A157237, A155860, A155904, A156695, A154257, A154285, A155114, A154536 Sequence in context: A146973 A003263 A271224 * A281423 A256626 A135211 Adjacent sequences:  A157239 A157240 A157241 * A157243 A157244 A157245 KEYWORD nice,nonn AUTHOR Zhi-Wei Sun, Feb 25 2009 STATUS approved

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