AlgebraDifficulty 7.9National olympiad, round 2Find the answer
A \emph{base 10 over-expansion} of a positive integer N is an expression of the form N=dk10k+dk−110k−1+⋯+d0100 with dk=0 and di∈{0,1,2,…,10} for all i. Which positive integers have a unique base 10 over-expansion?
A number or a short expression. Spacing and $ signs are ignored.
Solution
These are the integers with no 0's in their usual base 10 expansion. If the usual base 10 expansion of N is dk10k+⋯+d0100 and one of the digits is 0, then there exists an i≤k−1 such that di=0 and di+1>0; then we can replace di+110i+1+(0)10i by (di+1−1)10i+1+(10)10i to obtain a second base 10 over-expansion. We claim conversely that if N has no 0's in its usual base 10 expansion, then this standard form is the unique base 10 over-expansion for N. This holds by induction on the number of digits of N: if 1≤N≤9, then the result is clear. Otherwise, any base 10 over-expansion N=dk10k+⋯+d110+d0100 must have d0≡N(mod10), which uniquely determines d0 since N is not a multiple of 10; then (N−d0)/10 inherits the base 10 over-expansion dk10k−1+⋯+d1100, which must be unique by the induction hypothesis.
Want a route through all this instead of an archive? The track
puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.
Source: Omni-MATH,
licensed Apache-2.0.
Statement and solution reproduced as published; topic and difficulty added by this site.