Cost of a class period is a monotonically increasing function of the degree of interaction of that class period, cost(n) = g(DOI(n)). Total cost of a class COST is the sum of the costs of all class periods.
Teaching effectiveness (TE) of a class period is the sum of immediate teaching effectiveness of that class period and residue teaching effectiveness (RTE) defined as a fraction of the teaching effectiveness of the previous class period.
TE(n) = ITE(n) + RTE(n)
= ITE(n) + a TE(n-1)
= ITE(n) + a ITE(n-1) + a*2 ITE(n-2) + ... a*(n-1) ITE(1)