@Article{LanguascoZaccagnini2007a,
  Author    = "A. Languasco and A. Zaccagnini",
  Title     = "A note on {Mertens'} formula for arithmetic progressions",
  Journal   = "J. Number Theory",
  Volume    = 127,
  Pages     = "37--46",
  Year      = 2007,
  Abstract  = "We study the Mertens product over primes in arithmetic
               progressions, and find a uniform version of previous
               results on the asymptotic formula, improving at the
               same time the size of the error term, and giving an
               alternative, simpler value for the constant appearing
               in the main term."
}