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 A188903 a(n) is the least power of 2 such that 2n+1 - a(n) is prime, or 0 if no such prime exists. 3
 0, 1, 2, 2, 2, 4, 2, 2, 4, 2, 2, 4, 2, 4, 16, 2, 2, 4, 8, 2, 4, 2, 2, 4, 2, 4, 16, 2, 4, 16, 2, 2, 4, 8, 2, 4, 2, 2, 4, 8, 2, 4, 2, 4, 16, 2, 4, 16, 8, 2, 4, 2, 2, 4, 2, 2, 4, 2, 4, 16, 8, 16, 16, 0, 2, 4, 2, 4, 64, 2, 2, 4, 8, 8, 0, 2, 2, 4, 8, 2, 4, 32, 2, 4, 2, 4, 16, 2, 4, 16, 2, 2, 4, 8, 8, 64, 2, 2, 4, 2, 2, 4, 8, 8, 16, 32, 2, 4, 128, 8, 64, 32, 2, 4, 2, 2, 4, 2, 4, 16, 2, 2, 4, 8, 8, 0, 2 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS The second Polignac's Conjecture states that every odd positive integer is the sum of a prime and a power of two. This conjecture was proved false, and the smallest counterexample is 127 because subtracting powers of 2 from 127 produces the composite numbers 126, 123, 119, 111, 95, and 63. The sequence A006285 gives the odd numbers for which the conjecture fails. Hence, a(n) = 0 for n = (A006285(k)-1)/2 = {0, 63, 74, 125, 165, 168, 186, ...}. REFERENCES David Wells, Prime Numbers: The Most Mysterious Figures In Math, John Wiley & Sons, 2005, p. 175-176. LINKS Table of n, a(n) for n=0..126. Carlos Rivera, Puzzle 219, Polignac numbers, The Prime Puzzles and Problems Connection. EXAMPLE a(1) = 1 because 2*1 + 1 = 3 = 1 + 2 ; a(2) = 2 because 2*2 + 1 = 5 = 2 + 3 ; a(3) = 2 because 2*3 + 1 = 7 = 2 + 5 ; a(63) = 0 ; a(74) = 0 ; a(125) = 0, .... MAPLE with(numtheory):for n from 1 to 126 do:x:=2*n+1:id:=0:for k from 0 to 50 while(id=0) do: for q from 1 to 100 while(id=0) do: p:=ithprime(q): y:=2^k+p:if y=x then id:=1:printf(`%d, `, 2^k):else fi:od:od:if id=0 then printf(`%d, `, 0):else fi:od: MATHEMATICA Table[d = 2*n + 1; k = 1; While[k < d && ! PrimeQ[d - k], k = 2*k]; If[k < d, k, 0], {n, 0, 126}] PROG (Sage) def A188903(n): return next((2**k for k in (0..floor(log(2*n+1, 2))) if is_prime(2*n+1-2**k)), 0) # D. S. McNeil, Apr 14 2011 CROSSREFS Cf. A065381 (primes not of the form p + 2^k, p prime and k >= 0), A156695. Sequence in context: A367626 A049716 A361289 * A366534 A066671 A159802 Adjacent sequences: A188900 A188901 A188902 * A188904 A188905 A188906 KEYWORD nonn AUTHOR Michel Lagneau, Apr 13 2011 STATUS approved

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