famous papers/results by non professional mathematicians 
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What recent discoveries have amateur mathematicians made? 

Dear overflowers
Out of curiosity: do you know any famous papers and/or results by non professional mathematicians? (I realize that 'non professional mathematicians' is quite vague, so let's also say 'amateur'?)
Thank you for your answers!
Edit: I meant of course math papers. Also, I am interested in somewhat recent examples, say the last 2 centuries.
 A: Ramanujan was an autodidact and a clerk, so perhaps not really a professional mathematician.
His "results" are famous, and without any doubt extraordinary contributions to mathematics.
A: E.T. Bell, of course, dubbed Fermat the "prince of amateurs." 
A: Robert Ammann (from Wikipedia):

Robert Ammann [...] was an amateur mathematician who made several significant and groundbreaking contributions to the theory of quasicrystals and aperiodic tilings.
Ammann attended Brandeis University, but generally did not go to classes, and left after three years. He worked as a programmer for Honeywell. After ten years, his position was eliminated as part of a routine cutback, and Ammann ended up working as a mail sorter for a post office.
He discovered several new aperiodic tilings, each among the simplest known examples of aperiodic sets of tiles. He also showed how to generate tilings using lines in the plane as guides for lines marked on the tiles, now called "Ammann bars".
Ammann's discoveries came to notice only after Penrose had published his own discovery and gained priority.

He published one paper with Grünbaum and Shephard.
A: Kurt Heegner is a nice example. Sadly he died before the mathematical community realized that 
his proof of the class number 1 problem was essentially correct.
A: Let's start with all time classics.
A: Edward Witten is a theoretical physicist who has made important contributions to mathematics.  The controversial question may be whether to consider him a professional mathematician.  (Witten's Wikipedia entry quotes Michael Atiyah saying "Although he is definitely a physicist, his command of mathematics is rivaled by few mathematicians.")
A: The text below is quoted from a 1990 article in Scientific American dealing with the discovery of the Mandelbrot set. The emphasis at the end is mine, and the point (subject to debate) is that although Mandelbrot had a PhD in Mathematics, he did not do serious research related to his discovery of fractal structures. He is of course given due credit for the paradigm change he created.

Sullivan, who has also been acclaimed for his studies of the Mandelbrot set, calls himself "sort of a defender of Mandelbrot." Mandelbrot deserves to have the set named after him, Sullivan says, because his efforts brought the set to the attention of both the public and of the pure-mathematics community.
The fact that it was only "by coincidence" that the set proved later to be mathematically significant, Sullivan says, in no way diminishes Mandelbrot's achievement. "That's the wonderful thing about mathematics," he adds. "Even amateurs can make important contributions."

