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A094381 Number of numbers having exactly n representations as ab+ac+bc with 1 <= a <= b <= c. 4


%S 18,16,61,30,133,51,119,48,275,59,217,72,386,65,292,83,545,101,332,89,

%T 673,120,453,106,865,104

%N Number of numbers having exactly n representations as ab+ac+bc with 1 <= a <= b <= c.

%C Numbers up to 250,000 were checked. Note that there seem to be many more numbers having an even number of representations. Note that the Mathematica program computes A094379, A094380 and A094381, but outputs only this sequence.

%D See A025052

%e a(1) = 16 because there are 16 numbers (A093670) with unique representations.

%t cntMax=10; nSol=Table[{0, 0, 0}, {cntMax+1}]; Do[lim=Ceiling[(n-1)/2]; cnt=0; Do[If[n>a*b && Mod[n-a*b, a+b]==0 && Quotient[n-a*b, a+b]>=b, cnt++; If[cnt>cntMax, Break[]]], {a, 1, lim}, {b, a, lim}]; If[cnt<=cntMax, If[nSol[[cnt+1, 1]]==0, nSol[[cnt+1, 1]]=n]; nSol[[cnt+1, 2]]=n; nSol[[cnt+1, 3]]++;], {n, 10000}]; Table[nSol[[i, 3]], {i, cntMax+1}]

%Y Cf. A025052 (n having no representations), A093670 (n having one representation), A094379, A094380.

%K nonn

%O 0,1

%A _T. D. Noe_, Apr 28 2004

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Last modified March 29 15:03 EDT 2020. Contains 333107 sequences. (Running on oeis4.)