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 A157225 Number of ways to write the n-th positive odd integer in the form p+2^x+7*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, 1, 1, 0, 2, 1, 0, 2, 3, 1, 2, 4, 1, 2, 4, 2, 2, 3, 2, 2, 4, 2, 4, 4, 1, 5, 5, 2, 5, 7, 1, 3, 7, 2, 4, 8, 2, 4, 3, 2, 4 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,14 COMMENTS On Feb. 24, 2009, Zhi-Wei Sun conjectured that a(n)=0 if and only if n<11 or n=13,16,992; in other words, except for 25, 31, 1983, any odd integer greater than 20 can be written as the sum of a prime congruent to 5 mod 6, a positive power of 2 and seven times a positive power of 2. Sun verified the conjecture for odd integers below 5*10^7, and Qing-Hu Hou continued the verification for odd integers below 1.5*10^8 (on Sun's request). 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+7*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)=3 solutions are 2*18-1=5+2+7*2^2=5+2^4+7*2=17+2^2+7*2. MATHEMATICA PQ[x_]:=x>1&&Mod[x, 6]==5&&PrimeQ[x] RN[n_]:=Sum[If[PQ[2n-1-7*2^x-2^y], 1, 0], {x, 1, Log[2, (2n-1)/7]}, {y, 1, Log[2, Max[2, 2n-1-7*2^x]]}] Do[Print[n, " ", RN[n]], {n, 1, 200000}] CROSSREFS A000040, A000079, A157218, A155860, A155904, A156695, A154257, A154285, A155114, A154536, A154404, A154940 Sequence in context: A262781 A157218 A004718 * A055347 A055288 A203995 Adjacent sequences:  A157222 A157223 A157224 * A157226 A157227 A157228 KEYWORD nice,nonn AUTHOR Zhi-Wei Sun, Feb 25 2009 STATUS approved

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Last modified April 9 00:53 EDT 2020. Contains 333331 sequences. (Running on oeis4.)