Hello Math-Fun, a(1)=1 no duplicate term lexicographically first sequence such that a(n) + a(n+1) is pandigital: How many (positive) terms in S? S=1, 1023456788, 10, 1023456779, 19, 1023456770, 28, 102345661, 37,... (the -9 pattern between even indexed terms is obvious and so is the +9 pattern between the odd indexed ones) Best, É.