Exact PDF and CDF of a sum of Erlang random variables. It uses the findings in : Imran Shafique Ansari and Ferkan Yilmaz and Mohamed-Slim Alouini and Oguz Kucur: "New Results on the Sum of Gamma Random Variates With Application to the Performance of Wireless Communication Systems over Nakagami-m Fading Channels", https://arxiv.org/abs/1202.2576 . Usage shown in test.m
Tested on Matlab (R) R2017b. Requires the Symbolic Toolbox.