Common difference in the arithmetic progression S000800(n).
1, 2, 10, 18, 126, 270, 1470, 1890, 2310, 7770, 23100, 28980, 307230, 325710, 392700, 1178100, 1501500, 2192190, 3393390
1
T. D. Noe, Plot of 19 terms
(Mma) nn = 10000; sf = Select[Range[nn], SquareFreeQ]; mx = 0; t = {}; Do[diff = sf[[i]] - 1; k = 2; While[1 + k*diff <= nn && MemberQ[sf, 1 + k*diff], k++]; If[1 + k*diff > nn, Break[]]; If[1 + k*diff <= nn && k > mx, mx = k; AppendTo[t, {k, diff, 1 + (k - 1)*diff}]], {i, 2, nn}]; Transpose[t][[2]]
nonn,hard
T. D. Noe, Dec 16 2015