I have an iPad app that lets you draw equations with your finger
Windows 7 and 8 have a "Math Input Panel" in Accessories (actually it may be named slightly differently in Windows 8, I must check at home) in which you can use your mouse to enter the mathematical symbols. And better than a screen shot it handshakes directly into Word. I must check on my newly acquired Surface how it works with a finger input.
These have huge erases; I just got one for my niece since she went through 10 erasers in two weeks on her Kuro Toga. BTW, the Kuro Toga Roulette is the best mechanical pencil; it's metal, and rotates the lead every time you advance. I have the gunmetal version; I only wish Uni would make it in 0.3mm.
Most of the time, I prefer using pens, but I've been using pencils more after I got my Roulette and a Pentel GraphGear 1000. Another intesting option is Pilot's Frixion erasable pens; they actually work well, much better than the PaperMates of the late 1970's.