Is Bitcoin the first e-cash scheme which solved the double-spend problem? Is Bitcoin the first trustless e-cash scheme which solved the double-spend problem? I'm under the impression it is and that was one if its greatest contributions, but I couldn't really find a source to validate that claim. I know it's silly to ask for proof of something not existing, but I'm writing a thesis.
Yes. Bitcoin is the first trustless e-cash protocol, which solves the double-spend problem. If anyone else needs an academic source to back up this claim, there is at least this one:

Brito, Jerry ja Castillo, Andrea: Bitcoin: A primer for policymakers.
Mercatus Center at George Mason University, 2013

https://www.mercatus.org/publication/bitcoin-primer-policymakers