Mertens products in arithmetic progressions over function fields

We establish a function field analogue of Mertens’ formula for Euler products restricted to primes in arithmetic progressions over the polynomial ring F_q[t]. Our results are in direct correspondence with those of Languasco and Zaccagnini for arithmetic progressions in the integers. Over function fields, Weil’s Riemann hypothesis for Dirichlet L-functions holds unconditionally, and consequently the analogue of the GRH-strength asymptotic is obtained without any exceptional zero correction term.