We develop robust pricing and hedging of a weighted variance swap when market
prices for a finite number of co--maturing put options are given. We assume the
given prices do not admit arbitrage and deduce no-arbitrage bounds on the
weighted variance swap along with super- and sub- replicating strategies which
enforce them. We find that market quotes for variance swaps are surprisingly
close to the model-free lower bounds we determine. We solve the problem by
transforming it into an analogous question for a European option with a convex
payoff. The lower bound becomes a problem in semi-infinite linear programming
which we solve in detail. The upper bound is explicit.
We work in a model-independent and probability-free setup. In particular we
use and extend F\"ollmer's pathwise stochastic calculus. Appropriate notions of
arbitrage and admissibility are introduced. This allows us to establish the
usual hedging relation between the variance swap and the 'log contract' and
similar connections for weighted variance swaps. Our results take form of a
FTAP: we show that the absence of (weak) arbitrage is equivalent to the
existence of a classical model which reproduces the observed prices via
risk-neutral expectations of discounted payoffs.Comment: 25 pages, 4 figure
