3 Jun
2020
3 Jun
'20
4:52 p.m.
EA: "109,87,65,43,21,1,23,45,67,89. Is this the longest chain where the mean of two adjacent terms is given by the concatenation of the two digits framing the comma between them?" Apart from infinite chains starting with 11, 16, 22, 33, 44, 66, 77, 88, 99, & 106, I believe that it is. Note however that you may also replace the terminal 89 with 91.