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 A068307 From Goldbach problem: number of decompositions of n into a sum of three primes. 44
 0, 0, 0, 0, 0, 1, 1, 1, 2, 1, 2, 2, 2, 1, 3, 2, 4, 2, 3, 2, 5, 2, 5, 3, 5, 3, 7, 3, 7, 2, 6, 3, 9, 2, 8, 4, 9, 4, 10, 2, 11, 3, 10, 4, 12, 3, 13, 4, 12, 5, 15, 4, 16, 3, 14, 5, 17, 3, 16, 4, 16, 6, 19, 3, 21, 5, 20, 6, 20, 2, 22, 5, 21, 6, 22, 5, 28, 5, 24, 7 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,9 COMMENTS For even n > 2, a(n) = A061358(n-2). - Reinhard Zumkeller, Aug 08 2009 Vinogradov proved that every sufficiently large odd number is the sum of three primes. - T. D. Noe, Mar 27 2013 The two Helfgott papers show that every odd number greater than 5 is the sum of three primes (this is the Odd Goldbach Conjecture). - T. D. Noe, May 14 2013, N. J. A. Sloane, May 18 2013 LINKS Robert G. Wilson v, Table of n, a(n) for n = 1..36000 (first 10000 terms from T. D. Noe) H. A. Helfgott, Minor arcs for Goldbach's problem, arXiv:1205.5252 [math.NT], 2012. H. A. Helfgott, Major arcs for Goldbach's theorem, arXiv:1305.2897 [math.NT], 2013. H. A. Helfgott, The ternary Goldbach conjecture is true, arxiv:1312.7748 [math.NT], 2013. H. A. Helfgott, The ternary Goldbach problem, arXiv:1404.2224 [math.NT], 2014. Yannick Saouter, Checking the odd Goldbach conjecture up to 10^20, Math. Comp. 67 (222) (1998) 863-866. Eric W. Weinstein, MathWorld: Vinogradov's Theorem Wikipedia, Goldbach's conjecture. FORMULA a(n) = Sum_{k=1..floor(n/3)} Sum_{i=k..floor((n-k)/2)} A010051(i) * A010051(k) * A010051(n-i-k). - Wesley Ivan Hurt, Mar 26 2019 a(n) = [x^n y^3] Product_{k>=1} 1/(1 - y*x^prime(k)). - Ilya Gutkovskiy, Apr 18 2019 EXAMPLE a(6) = 1 because 6 = 2+2+2, a(9) = 2 because 9 = 2+2+5 = 3+3+3, a(15) = 3 because 15 = 2+2+11 = 3+5+7 = 5+5+5, a(17) = 4 because 17 = 2+2+13 = 3+3+11 = 3+7+7 = 5+5+7. - Zak Seidov, Jun 29 2017 MATHEMATICA f[n_] := Block[{c = 0, lmt = PrimePi@ Floor[n/2], p, q}, Do[p = Prime@ i; q = Prime@ j; r = n - p - q; If[ PrimeQ@ r && r >= p, c++ ], {i, lmt}, {j, i}]; c]; Array[f, 91] (* Robert G. Wilson v, Apr 13 2008 *) PROG (PARI) a(n)=my(s); forprime(p=(n+2)\3, n-4, forprime(q=(n-p+1)\2, min(n-p-2, p), if(isprime(n-p-q), s++))); s \\ Charles R Greathouse IV, Jun 29 2017 (Python) from sympy import isprime, primerange, floor def a(n):     s=0     for p in primerange(floor((n + 2)/3), n - 3):         for q in primerange(floor((n - p + 1)/2), min(n - p - 2, p) + 1):             if isprime(n - p - q): s+=1     return s print [a(n) for n in xrange(1, 101)] # Indranil Ghosh, Jul 01 2017, after PARI code by Charles R Greathouse IV CROSSREFS Bisections: A045917, A054860. Cf. A002375, A007963, A061358, A059998. First occurrence: A139321. Records: A139322. Column k=3 of A117278. Sequence in context: A060764 A105149 A295894 * A158946 A303428 A223853 Adjacent sequences:  A068304 A068305 A068306 * A068308 A068309 A068310 KEYWORD easy,nonn AUTHOR Naohiro Nomoto, Feb 24 2002 EXTENSIONS More terms from Vladeta Jovovic, Mar 10 2002 STATUS approved

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Last modified August 23 11:55 EDT 2019. Contains 326222 sequences. (Running on oeis4.)