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jmc

number theory senior

Problem

The Lucas numbers are given by the recursion , and . Find the units digit of .
Solution
First, we need to find . We find that Thus, . To find its units digit, we continue listing more values in the sequence until we reach a pattern: the units digit of is that of , and so is ; that of is ; and that of is . Hence, the units digit repeats starting from here, with a period of . As , then the units digit of is the same as that of , or .
Final answer
4