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 A211190 Number of ways to write 2n = p+2q+3r with p,q,r terms of A210479 1
 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 2, 2, 3, 3, 3, 4, 4, 3, 4, 3, 3, 3, 4, 4, 5, 5, 5, 5, 4, 7, 6, 6, 7, 5, 6, 7, 7, 7, 7, 5, 5, 8, 6, 7, 8, 5, 8, 10, 9, 9, 11, 9, 8, 12, 9, 8, 10, 7, 7, 10, 8, 7, 9, 7, 6, 12, 8, 9, 11, 7, 8, 10, 8, 7, 11, 8, 7, 11, 7, 7, 10, 6, 5, 8, 7, 6, 10, 7, 7, 10, 7, 6, 11, 7, 7, 10, 5, 5, 10, 5 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,11 COMMENTS Conjecture: a(n)>0 for all n>8. Moreover, for positive integers a<=b<=c, all integers n>=3(a+b+c) with n-a-b-c even can be written as a*p+b*q+c*r with p,q,r terms of A210479, if and only if (a,b,c) is among the following 6 triples: (1,2,3), (1,2,4), (1,2,8), (1,2,9), (1,3,5), (1,3,8). The author also conjectured that if n>8 is odd, different from 201 and 447, and not congruent to 1 or -1 modulo 12, then n can be written as a sum of three terms of A210479. LINKS Zhi-Wei Sun, Table of n, a(n) for n = 1..5000 G. Melfi, On two conjectures about practical numbers, J. Number Theory 56 (1996) 205-210 [MR96i:11106]. Zhi-Wei Sun, Sandwiches with primes and practical numbers, a message to Number Theory List, Jan. 13, 2013. Zhi-Wei Sun, Conjectures involving primes and quadratic forms, arXiv:1211.1588 [math.NT], 2012-2017. EXAMPLE a(10)=1 since 2*10=5+2*3+3*3 with 3 and 5 terms of A210479. MATHEMATICA f[n_]:=f[n]=FactorInteger[n] Pow[n_, i_]:=Pow[n, i]=Part[Part[f[n], i], 1]^(Part[Part[f[n], i], 2]) Con[n_]:=Con[n]=Sum[If[Part[Part[f[n], s+1], 1]<=DivisorSigma[1, Product[Pow[n, i], {i, 1, s}]]+1, 0, 1], {s, 1, Length[f[n]]-1}] pr[n_]:=pr[n]=n>0&&(n<3||Mod[n, 2]+Con[n]==0) p[k_]:=p[k]=pr[Prime[k]-1]==True&&pr[Prime[k]+1]==True q[n_]:=q[n]=PrimeQ[n]==True&&pr[n-1]==True&&pr[n+1]==True a[n_]:=a[n]=Sum[If[p[j]==True&&p[k]==True&&q[2n-2Prime[j]-3Prime[k]]==True, 1, 0], {j, 1, PrimePi[n]}, {k, 1, PrimePi[(2n-2Prime[j])/3]}] Do[Print[n, " ", a[n]], {n, 1, 100}] CROSSREFS Cf. A005153, A210479, A210480, A210681, A211165. Sequence in context: A086712 A125842 A029865 * A072644 A179864 A070082 Adjacent sequences:  A211187 A211188 A211189 * A211191 A211192 A211193 KEYWORD nonn AUTHOR Zhi-Wei Sun, Feb 03 2013 STATUS approved

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Last modified July 30 21:33 EDT 2021. Contains 346365 sequences. (Running on oeis4.)