<a href="http://www.wolframalpha.com/input/?i=what+is+the+formula+for+stellar+magnitudes#">http://www.wolframalpha.com/input/?i=what+is+the+formula+for+stellar+magnitudes#</a>

It gave me a formula for comparing two magnitudes. not a definition which is more what I had in mind.  But this answer included a web tool for doing the calculation!

So I'm pretty impressed by this Wolfram.  Does this site graph and solve as well as Mathematica does?

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<a href="http://schoolblogs.cuip.net/2013studentallstarstwo/wp-content/uploads/sites/18/2013/07/2013-07-22-13_06_39-Clipboard-e1374516398527.png"><img class="alignnone size-medium wp-image-233" alt="2013-07-22 13_06_39-Clipboard" src="http://schoolblogs.cuip.net/2013studentallstarstwo/wp-content/uploads/sites/18/2013/07/2013-07-22-13_06_39-Clipboard-300x261.png" width="300" height="261" /></a>