[seqfan] Difference is a product of digits

[Eric Angelini]

Hello SeqFans,

Term a(n+1) of S is separated from a(n) by the difference "d"; 
"d" must be the product of a digit belonging to a(n) and a 
digit belonging to a(n+1).

Example:
S = 4, 12, 28, 34, 46, 74, 82, ...

from  4 to 12 there is 8  (that is 4x2)
from 12 to 28 there is 16 (that is 2x8)
from 28 to 34 there is 6  (that is 2x3)
from 34 to 46 there is 12 (that is 3x4)
from 46 to 74 there is 28 (that is 4x7)
from 74 to 82 there is 8  (that is 4x2)
...

One has sometimes multiple choices for a(n+1):

135, 136 (diff = 1x1)
135, 138 (diff = 1x3)
135, 147 (diff = 3x4)
135, 150 (diff = 3x5)
135, 165 (diff = 5x6)
135, 170 (diff = 5x7)

I've explored this together with Jean-Marc Falcoz.
Here are some results (and open questions, at the end):
http://www.cetteadressecomportecinquantesignes.com/DifProdDigits.htm
Best,
É.