Talk:2.9.4/@comment-5149782-20120616222858/@comment-140.180.189.95-20120617003615

The original solution interprets the "digits" of an integer a as the numbers n_i in the decimal expansion a = n_0 + 10*n_1 + 10*n_2 +...; using that definition of "digits", the proposition in the question is certainly true for all integers. The two proofs given for nonnegative integers are exactly the same, so it probably makes sense to merge the two.