OFFSET

1,1

EXAMPLE

As a(1) = 2 and a(2) = 3, the product 2 * 3 = 6 cannot be in the sequence;

as a(2) = 3 and a(3) = 4, the product 3 * 4 = 12 cannot be in the sequence;

(...)

as a(6) = 8 and a(7) = 9, the product 8 * 9 = 72 cannot be in the sequence;

as a(8) = 10, the product 1 * 0 = 0 cannot be in the sequence;

as a(9) = 11, the product 1 * 1 = 1 cannot be in the sequence; etc.

MATHEMATICA

lst={}; a[1]=2; a[n_]:=a[n]=Block[{k=2}, While[s=Array[a, n-1]; AppendTo[lst, p=Times@@(Flatten[IntegerDigits/@Join[Last@s, {k}]][[-2;; ]])]; MemberQ[s, p]||MemberQ[s, k]||MemberQ[lst, k], k++; lst=Most@lst]; k]; Array[a, 73] (* Giorgos Kalogeropoulos, May 12 2022 *)

CROSSREFS

KEYWORD

base,nonn

AUTHOR

Eric Angelini and Jean-Marc Falcoz, Jul 27 2020

STATUS

approved