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Least number one short of every divisor from 2 to 10

Question

Find the smallest positive integer that leaves remainder nn on division by n+1n+1, for every nn from 11 to 99 (so remainder 11 mod 22, remainder 22 mod 33, up to remainder 99 mod 1010).

Short answer

25192519. Each condition says X≡−1(modn+1)X\equiv -1 \pmod{n+1}, so X+1X+1 is divisible by all of 2,…,102,\dots,10. The least such X+1X+1 is lcm⁡(2,…,10)=2520\operatorname{lcm}(2,\dots,10)=2520, giving X=2519X=2519.

Adapted from QuantQuestions.io; question and answer rewritten by SelfMadeQuant.com.

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