OFFSET
1,1
COMMENTS
Triangular number with index given by A095863.
MATHEMATICA
Join[{6}, (#(#+1))/2&/@Table[Floor[(Sqrt[8*10^n+1]-1)/2], {n, 2, 30}]] (* Harvey P. Dale, Jul 27 2012 *)
triInverse[n_] := Floor[(Sqrt[1 + 8*n] - 1)/2]; tri[n_] := n*(n+1)/2; Table[tri[triInverse[10^n - 1]], {n, 18}] (* T. D. Noe, Jul 27 2012 *)
CROSSREFS
KEYWORD
base,easy,nonn
AUTHOR
Ray Chandler, Jun 28 2004
STATUS
approved