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 A045917 From Goldbach problem: number of decompositions of 2n into unordered sums of two primes. 86
 0, 1, 1, 1, 2, 1, 2, 2, 2, 2, 3, 3, 3, 2, 3, 2, 4, 4, 2, 3, 4, 3, 4, 5, 4, 3, 5, 3, 4, 6, 3, 5, 6, 2, 5, 6, 5, 5, 7, 4, 5, 8, 5, 4, 9, 4, 5, 7, 3, 6, 8, 5, 6, 8, 6, 7, 10, 6, 6, 12, 4, 5, 10, 3, 7, 9, 6, 5, 8, 7, 8, 11, 6, 5, 12, 4, 8, 11, 5, 8, 10, 5, 6, 13, 9, 6, 11, 7, 7, 14, 6, 8, 13, 5, 8, 11, 7, 9 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,5 COMMENTS Note that A002375 (which differs only at the n = 2 term) is the main entry for this sequence. The graph of this sequence is called Goldbach's comet. - David W. Wilson, Mar 19 2012 This is the row length sequence of A182138, A184995 and A198292. - Jason Kimberley, Oct 03 2012 The Goldbach conjecture states that a(n) > 0 for n >= 2. - Wolfdieter Lang, May 14 2016 With the second Maple program, the command G(2n) yields all the unordered pairs of prime numbers having sum 2n; caveat: a pair {a,a} is listed as {a}. Example: G(26) yields {{13}, {3,23}, {7,19}}. The command G(100000) yields 810 pairs very fast. - Emeric Deutsch, Jan 03 2017 Conjecture: Let p denote any prime in any decomposition of 2n. 4 and 6 are the only numbers n such that 2n + p is prime for every p. - Ivan N. Ianakiev, Apr 06 2017 Conjecture: For all m >= 0, there exists at least one possible value of n such that a(n) = m. - Ahmad J. Masad, Jan 06 2018 REFERENCES Calvin C. Clawson, "Mathematical Mysteries, the beauty and magic of numbers," Perseus Books, Cambridge, MA, 1996, Chapter 12, pages 236-257. H. Halberstam and H. E. Richert, 1974, "Sieve methods", Academic press, London, New York, San Francisco. LINKS H. J. Smith, Table of n, a(n) for n = 1..20000 M. Herkommer, Goldbach Conjecture Research Eric Weisstein's World of Mathematics, Goldbach Partition Wikipedia, Goldbach's conjecture G. Xiao, WIMS server, Goldbach FORMULA From Halberstam and Richert: a(n) < (8+0(1))*c(n)*n/log(n)^2 where c(n) = Product_{p>2} (1 - 1/(p-1)^2)*Product_{p|n, p>2} (p-1)/(p-2). It is conjectured that the factor 8 can be replaced by 2. - Benoit Cloitre, May 16 2002 a(n) = ceiling(A035026(n) / 2) = (A035026(n) + A010051(n)) / 2. a(n) = Sum_{i = 2..n} floor( 2/Omega(i * (2*n-i) ) ). - Wesley Ivan Hurt, Jan 24 2013 a(n) = A224709(n) + (primepi(2n-2) - primepi(n-1)) + primepi(n) + 1 - n. - Anthony Browne, May 03 2016 a(n) = A224708(2n) - A224708(2n+1) + A010051(n). - Anthony Browne, Jun 26 2016 MAPLE A045917 := proc(n)     local a, i ;     a := 0 ;     for i from 1 to n do         if isprime(i) and isprime(2*n-i) then             a := a+1 ;         end if;     end do:     a ; end proc: # R. J. Mathar, Jul 01 2013 # second Maple program: G := proc (n) local g, j: g := {}: for j from 2 to (1/2)*n do if isprime(j) and isprime(n-j) then g := `union`(g, {{n-j, j}}) end if end do: g end proc: seq(nops(G(2*n)), n = 1 .. 98); # Emeric Deutsch, Jan 03 2017 MATHEMATICA f[n_] := Length[Select[2n - Prime[Range[PrimePi[n]]], PrimeQ]]; Table[ f[n], {n, 100}] (* Paul Abbott, Jan 11 2005 *) nn = 10^2; ps = Boole[PrimeQ[Range[1, 2*nn, 2]]]; Join[{0, 1}, Table[Sum[ps[[i]] ps[[n-i+1]], {i, Ceiling[n/2]}], {n, 3, nn}]] (* T. D. Noe, Apr 13 2011 *) PROG (PARI) a(n)=my(s); forprime(p=2, n, s+=isprime(2*n-p)); s \\ Charles R Greathouse IV, Mar 27 2012 (Haskell) a045917 n = sum \$ map (a010051 . (2 * n -)) \$ takeWhile (<= n) a000040_list -- Reinhard Zumkeller, Sep 02 2013 (Python) from sympy import isprime def A045917(n): ....x = 0 ....for i in range(2, n+1): ........if isprime(i) and isprime(2*n-i): ............x += 1 ....return x # Chai Wah Wu, Feb 24 2015 CROSSREFS Cf. A002375 (the main entry for this sequence (which differs only at the n=2 term)). Cf. A023036 (first appearance of n), A000954 (last (assumed) appearance of n). Cf. also A000040, A182138, A184995, A185297, A187129, A010051, A198292, A035026, A224708, A224709. Sequence in context: A230443 A254610 A002375 * A240708 A235645 A325357 Adjacent sequences:  A045914 A045915 A045916 * A045918 A045919 A045920 KEYWORD easy,nice,nonn,look AUTHOR STATUS approved

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Last modified October 22 22:34 EDT 2019. Contains 328335 sequences. (Running on oeis4.)