OFFSET
1,1
COMMENTS
Number of prime factors of a(n): 2, 2, 3, 5, 3, 4, 5, 4, 5, 7, 5, 6, 4, 4, 7, 5, 6, 4, 5, 5, 5, 10, 6, 11, 4, 7, 8, 9, 8, 7. Are a(1) = 6 and a(2) = 91 the only semiprimes?
EXAMPLE
6 = 2*3, 91 = 7*13.
MATHEMATICA
t[n_] := n(n+1)/2; Table[m = 2*10^k // Sqrt // Floor; Select[Table[t[x], {x, m + 1, m - 20, -1}], SquareFreeQ[#] && # < 10^k &, 1][[1]], {k, 30}]
lsftr[i_]:=Module[{g=Floor[(Sqrt[1+8*10^i]-1)/2]}, While[ !SquareFreeQ[ (g(g+1))/2], g--]; (g(g+1))/2]; Join[{6}, Array[lsftr, 20, 2]] (* Harvey P. Dale, Jun 19 2013 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Zak Seidov, Jun 05 2013
STATUS
approved