sum[i=1 to n: f(n,t)]  	sum of a series
Σ[i=1 to n: f(n,t)]  	sum of a series

prod[t<ti≤t+θ: f]			product of series of function f from term t through t+θ
Π[t<ti≤t+θ: f]				product of series of function f from term t through t+θ

http://www.ams.org/journals/proc/1957-008-05/S0002-9939-1957-0090919-0/S0002-9939-1957-0090919-0.pdf 
example of limit derivative : 
D_plus		D_plus(t)  = lim{s->t_plus : (r(t,t) - r(s,t))/(t - s)) }
D_minus		D_minus(t) = lim{s->t_minus: (r(t,t) - r(s,t))/(t - s)) }