Can I publish a paper on my PhD work without informing my old supervisor? I was a PhD student and working on a paper. My adviser told me that he could not support me anymore so I left him. Now I am wondering if I could publish that paper on my own. There were 3 other coauthors on that paper but I was the first author and main idea was from me.
Your paper had other authors. Generally, if you want to publish it, you have to communicate with the other co-authors (whether they were your supervisor or not).

Regardless of who the first author on a paper is, if other individuals made authorship-worthy contributions to the work, you can't publish it without communicating with them first.