1,2,3,4,5,6,7,8,9,100,111,122,133,144,155,166,177,188,199,200,221,242,263,284,205,226,247,268,289,300,331,362,393,324,355,386,317,348,379,400,441,482,423,464,405,446,487,428,469,500,551,502,553,504,555,506

def a(n):
    s = str(n)
    if len(s) < 2: return n
    res = []
    for i in range(len(s) - 1):
        d1, d2 = int(s[i]), int(s[i+1])
        res.append(s[i])
        res.append(str((d1 * d2) % 10))
    res.append(s[-1])
    return int("".join(res))
print([a(n) for n in range(1, 57)])

a[n_]:=Module[{d=IntegerDigits[n]}, l=Length[d]; If[l==1, n, Do[j=2i; d=Insert[d, Mod[d[[j-1]]*d[[j]], 10], j], {i, l-1}]; FromDigits[d]]]; Array[a, 56]