A geometric sequence is a sequence, such that each term is given by a multiple of q of the previous one. A geometric series consists out of the sum of all former values of a geometric sequence..
# NOT RUN {GeomSn(a1=3, q=2, n=5)
# calculates the sum of the first 5 elements of the sequence(gseq <- 3 * (2^(0:5)))
sum(gseq)
GeomSn(a1=3, q=2, n=0:5)
# }