TITLE: Diophantine equation - $a^4+b^4=c^4+d^4$ ($a,b,c,d > 0$)
QUESTION [7 upvotes]: How can I find the general solution of $a^4+b^4=c^4+d^4$ ($a,b,c,d > 0$)?
And how did Euler find the solution $158^4+59^4=133^4+134^4$?

REPLY [2 votes]: some material related to your question can be found here : https://sites.google.com/site/tpiezas/013