Editing Ramanujan graph (section)

{{Short description|Spectral graph theory concept}}
In the mathematical field of [[spectral graph theory]], a '''Ramanujan graph''' is a [[regular graph]] whose [[spectral gap]] is almost as large as possible (see [[extremal graph theory]]). Such graphs are excellent [[expander graph|spectral expanders]]. As Murty's survey paper<ref>[http://www.mast.queensu.ca/~murty/ramanujan.pdf Survey paper by M. Ram Murty]</ref> notes, Ramanujan graphs "fuse diverse branches of pure mathematics, namely, [[number theory]], [[representation theory]], and [[algebraic geometry]]". 
These graphs are indirectly named after [[Srinivasa Ramanujan]]; their name comes from the [[Ramanujan–Petersson conjecture]], which was used in a construction of some of these graphs.