1. De binis formulis speciei xx + myy et xx + nyy inter se concordibus et discordibus
https://math.dartmouth.edu/~euler/docs/originals/E758.pdf
2. Variations on a Theme of Euler;Ono,1994
3. Euler’s Concordant Forms;Ono;Acta Arith.,1996
4. Concordant Numbers within Arithmetic Progressions and Elliptic Curves;Im;Proc. Amer. Math. Soc.,2013
5. http://www.math.uconn.edu/kconrad/ross2007/4squarearithprog.pdf