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 A156695 Odd numbers that are not of the form p + 2^a + 2^b, a, b > 0, p prime. 51
 1, 3, 5, 6495105, 848629545, 1117175145, 2544265305, 3147056235, 3366991695, 3472109835, 3621922845, 3861518805, 4447794915, 4848148485, 5415281745, 5693877405, 6804302445, 7525056375, 7602256605, 9055691835, 9217432215 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Crocker shows that this sequence is infinite. All members above 5 found so far (up to 2.5 * 10^11) are divisible by 255 = 3 * 5 * 17, and many are divisible by 257. I conjecture that all members of this sequence greater than 5 are divisible by 255. This implies that all odd numbers (greater than 7) are the sum of a prime and at most three positive powers of two. Pan shows that, for every c > 1, a(n) << x^c. More specifically, there are constants C,D > 0 such that there are at least Dx/exp(C log x log log log log x/log log log x) members of this sequence up to x. - Charles R Greathouse IV, Apr 11 2016 All terms > 5 are numbers k > 3 such that k - 2^n is a de Polignac number (A006285) for every n > 0 with 2^n < k. Are there numbers K such that |K - 2^n| is a Riesel number (A101036) for every n > 0? If so, ||K - 2^n| - 2^m| is composite for every pair m,n > 0, by the dual Riesel conjecture. - Thomas Ordowski, Jan 06 2024 LINKS Giovanni Resta, Table of n, a(n) for n = 1..233 (terms < 10^12) Roger Crocker, "On the sum of a prime and of two powers of two", Pacific Journal of Mathematics 36:1 (1971), pp. 103-107. Roger Crocker, Some counter-examples in the additive theory of numbers, Master's thesis (Ohio State University), 1962. Hao Pan, On the integers not of the form p + 2^a + 2^b. arXiv:0905.3809 [math.NT], 2009. Zhi-Wei Sun, Mixed sums of primes and other terms (2009-2010). EXAMPLE Prime factorization of terms: F_0 = 3, F_1 = 5, F_2 = 17, F_3 = 257 are Fermat numbers (cf. A000215) 6495105 = 3 * 5 * 17 * 25471 848629545 = 3 * 5 * 17 * 461 * 7219 1117175145 = 3 * 5 * 17 * 257 * 17047 2544265305 = 3^2 * 5 * 17 * 257 * 12941 3147056235 = 3^2 * 5 * 17 * 257 * 16007 3366991695 = 3 * 5 * 17 * 83 * 257 * 619 3472109835 = 3 * 5 * 17 * 257 * 52981 3621922845 = 3 * 5 * 17^2 * 257 * 3251 3861518805 = 3^3 * 5 * 17 * 257 * 6547 4447794915 = 3^3 * 5 * 17 * 257 * 7541 4848148485 = 3^4 * 5 * 17 * 704161 5415281745 = 3 * 5 * 17 * 21236399 5693877405 = 3^2 * 5 * 17 * 257 * 28961 6804302445 = 3^2 * 5 * 17 * 53 * 257 * 653 7525056375 = 3^2 * 5^3 * 17 * 257 * 1531 7602256605 = 3 * 5 * 17 * 257 * 311 * 373 9055691835 = 3 * 5 * 17 * 257 * 138181 9217432215 = 3^2 * 5 * 17 * 173 * 257 * 271 PROG (PARI) is(n)=if(n%2==0, return(0)); for(a=1, log(n)\log(2), for(b=1, a, if(isprime(n-2^a-2^b), return(0)))); 1 \\ Charles R Greathouse IV, Nov 27 2013 (Python) from itertools import count, islice from sympy import isprime def A156695_gen(startvalue=1): # generator of terms >= startvalue for n in count(max(startvalue+(startvalue&1^1), 1), 2): l = n.bit_length()-1 for a in range(l, 0, -1): c = n-(1<

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Last modified May 19 04:28 EDT 2024. Contains 372666 sequences. (Running on oeis4.)