Last term in the arithmetic progression listed in S000800(n).
3, 7, 71, 271, 2899, 11611, 67621, 92611, 173251, 823621, 2702701, 3419641, 39018211, 48856501, 64795501, 196742701, 267267001, 550239691, 956935981
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][[3]]
nonn,hard
T. D. Noe, Dec 16 2015