Click here to purchase the entire book in PDF format. Sigma - $\Sigma$ A $\Sigma$ (the Greek capital letter Sigma) in an equation is just a lazy way of writing ``the sum of'' whatever follows it. The stuff under and over it give you an indication of when to start and when to stop adding. Let's look at a simple example shown in Equation 1.78... $$y = \sum_{x=1}^{10} x$$ (2.78) Equation 1.78 says ``$y$ equals the sum of all of the values of $x$ from $x=1$ to $x=10$. The $\Sigma$ sign says ``add up everything from...'' the ``$x=1$'' at the bottom says where to start adding, the ``10'' on top says when to stop, and the ``$x$'' after the $\Sigma$ tells you what you're adding. So: $$\sum_{x=1}^{10} x = 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 = 55$$ (2.79) This, of course, can get a little more complicated, as is shown in Equation 1.80 but if you don't panic, it should be reasonably easy to figure out what you're adding. $$\sum_{x=1}^{5} \sin (x) = \sin (1) + \sin (2) + \sin (3) + \sin (4) + \sin (5)$$ (2.80)