OFFSET
1,1
COMMENTS
Conjecture: (i) a(n) exists for any n > 0. Also, the main term of a(n) is n^2/2 as n tends to the infinity.
(ii) No term a(n) with n>2 is congruent to 1 or -1 modulo 5.
See also the comments in A254595 for a similar conjecture.
LINKS
Zhi-Wei Sun, Table of n, a(n) for n = 1..200
Zhi-Wei Sun, On universal sums of polygonal numbers, arXiv:0905.0635 [math.NT], 2009-2015.
EXAMPLE
a(4) = 20 since 20 is the first positive integer m with A254574(m) = 4. Note that 20 = 0*1/2 + 3*(3*3+1)/2 + 2*(3*2-1)/2 = 1*2/2 + 2*(3*2+1)/2 + 3*(3*3-1)/2 = 3*4/2 + 1*(3*1+1)/2 + 3*(3*3-1)/2 = 5*6/2 + 0*(3*0+1)/2 + 2*(3*2-1)/2.
MATHEMATICA
TQ[n_]:=IntegerQ[Sqrt[8n+1]]
Do[Do[m=0; Label[aa]; m=m+1; r=0; Do[If[TQ[m-y(3y+1)/2-z(3z-1)/2], r=r+1; If[r>n, Goto[aa]]], {y, 0, (Sqrt[24m+1]-1)/6}, {z, 0, (Sqrt[24(m-y(3y+1)/2)+1]+1)/6}];
If[r==n, Print[n, " ", m]; Goto[bb], Goto[aa]]]; Label[bb]; Continue, {n, 1, 60}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Zhi-Wei Sun, Feb 03 2015
STATUS
approved