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 A157218 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 1 mod 6 and x,y positive integers. 3
 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 0, 2, 1, 0, 2, 3, 1, 1, 3, 1, 1, 4, 2, 3, 2, 1, 3, 3, 2, 3, 5, 1, 2, 5, 2, 4, 5, 1, 4, 3, 1, 4, 7, 1, 5, 7, 2 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,15 COMMENTS On Feb 24 2009, Zhi-Wei Sun conjectured that a(n)>0 for all n=18,19,...; in other words, any odd integer greater than 34 can be written as the sum of a prime congruent to 1 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 R. 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. LINKS Zhi-Wei Sun, Table of n, a(n) for n=1..200000 Z. W. Sun, Mixed sums of primes and other terms, preprint, 2009. arXiv:0901.3075 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 and M.-H. Le, Integers not of the form c*(2^a + 2^b) + p^{alpha}, Acta Arith. 99(2001), 183-190. FORMULA a(n) = |{: p+2^x+7*2^y=2n-1 with p a prime congruent to 1 mod 6 and x,y positive integers}|. EXAMPLE For n=19 the a(19)=3 solutions are 2*19 - 1 = 7 + 2 + 7*2^2 = 7 + 2^4 + 7*2 = 19 + 2^2 + 7*2. MATHEMATICA PQ[x_]:=x>1&&Mod[x, 6]==1&&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, A155860, A155904, A156695, A154257, A154285, A155114, A154536, A154404, A154940. Sequence in context: A145579 A167655 A262781 * A004718 A157225 A055347 Adjacent sequences:  A157215 A157216 A157217 * A157219 A157220 A157221 KEYWORD nice,nonn AUTHOR Zhi-Wei Sun, Feb 25 2009 STATUS approved

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Last modified September 19 02:57 EDT 2019. Contains 327186 sequences. (Running on oeis4.)