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 A119345 Numbers having exactly one representation as sum of two triangular numbers. 6
 0, 1, 2, 3, 4, 7, 9, 10, 11, 12, 13, 15, 18, 20, 22, 24, 25, 27, 28, 29, 30, 34, 37, 38, 39, 43, 45, 48, 49, 57, 58, 60, 61, 64, 65, 67, 69, 70, 73, 78, 79, 83, 84, 87, 88, 90, 92, 93, 97, 99, 100, 101, 102, 105, 108, 110, 112, 114, 115, 119, 127, 130, 132, 135, 137, 139, 142 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS A052343(a(n)) = 1; gives A020756 together with A118139. LINKS T. D. Noe, Table of n, a(n) for n = 1..1000 MATHEMATICA trn=SortBy[{First[#], Last[#], Total[#]}& /@ (Union[Sort/@Tuples[Accumulate[Range[0, 70]], {2}]]), Last]; Take[With[{x=Transpose[trn][[3]]}, Complement[Union[x], Union[Flatten[Select[Split[x], Length[#]>1&]]]]], 70]  (* Harvey P. Dale, Feb 14 2011 *) nn=100; tri=Table[n(n+1)/2, {n, 0, nn}]; sums=Select[Flatten[Table[tri[[i]]+tri[[j]], {i, nn}, {j, i}]], #

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Last modified June 6 10:22 EDT 2020. Contains 334853 sequences. (Running on oeis4.)