MathDB
2021 Team P4

Source:

March 2, 2021
combinatorics

Problem Statement

How many four-digit positive integers a1a2a3a4‾\overline{a_1a_2a_3a_4} have only nonzero digits and have the property that ∣ai−aj∣≠1|a_i-a_j| \neq 1 for all 1≤i<j≤4?1 \leq i<j \leq 4?
Proposed by Kyle Lee
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