MathDB
Different sums mod n_1

Source: Romania TST 1 2012, Problem 1

May 3, 2012
inductionnumber theorygreatest common divisorgeometrynumber theory proposed

Problem Statement

Let n1,,nkn_1,\ldots,n_k be positive integers, and define d1=1d_1=1 and di=(n1,,ni1)(n1,,ni)d_i=\frac{(n_1,\ldots,n_{i-1})}{(n_1,\ldots,n_{i})}, for i{2,,k}i\in \{2,\ldots,k\}, where (m1,,m)(m_1,\ldots,m_{\ell}) denotes the greatest common divisor of the integers m1,,mm_1,\ldots,m_{\ell}. Prove that the sums i=1kaini\sum_{i=1}^k a_in_i with ai{1,,di}a_i\in\{1,\ldots,d_i\} for i{1,,k}i\in\{1,\ldots,k\} are mutually distinct modn1\mod n_1.