In quantitative finance, my professor posted solution to our homework in the following spread sheet. I know what S, K, T, sigma, rho, delta, gamma (the greeks) are. What I don't understand are the letters N, U, A, Q, and df. What are these short for? I will also include the question below if that helps.

#2. Build in Excel a four-step tree to value a six-month European call on a non-dividend paying stock.

Assume the current stock price is 40, the call strike is 40, the annualized risk-free rate is 12%, and the annualized volatility of the stock is 40%.

stock call
S	40
K	40
T	0.5
sigma	0.4
r	12%
n	4
u	1.15191
d	0.868123
a	1.015113
q	0.5180
df	0.9851