Ue B 25.5 (c) How do I create a sigma notation for this series? My suggestion: 1 + Sigma (from infinity to n=0) theta^n/ ((2^n) * n!) I have not included the changes of the signs, i.e. + + - - + ... I know that I can show alternative changes with (-1)^n but here I do not know how to handle the sign changes. Is my double factorial correct? How (when) can I express a double factorial in a single factorial