PlainCalc

From the author of Print2PDF (an absolute necessity in the pre Mac OS X days), comes an amazing little gem for us math geeks.

From the website:

PlainCalc is a buttonless, algebraic-entry, mathematical calculator with variables. To use PlainCalc, type an expression or assignment and press Return or Enter. Unlike most calculators I've seen, PlainCalc allows you to indicate multiplication by juxtaposition, e.g., you can write 2πr instead of 2*π*r.

And:

You can define your own functions, and you can save worksheets, including values of variables and functions. Version 2 requires OS 10.4 or later. This version was built as a universal binary. 

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