[seqfan] Two neighboring digits multiply to a composite

[Eric Angelini]

More on the same idea:

Smallest integer not yet in C such that
two neighboring digits multiply to a
composite:

C = 1,4,2,3,4,5,6,7,8,9,14,16,18,19,22,
23,24,25,26,27,28,29,32,33,34,35,36,
37,38,39,41,42,43,44,45,46,47,48,49,
52,53,54,55,56,57,58,59,61,62,63,64,
65,66,67,68,69,72,73,74,75,76,77,78,
79,81,82,83,84,85,86,87,88,89,91,92,
93,94,95,96,97,98,99,141,414,161,...

Best,
É.

(and what about Z: smallest integer
not yet present in Z such that two
neighboring digits don't multiply to
a prime nor a composite. ;-)