What exactly are the “bounds” in arbitrage bounds?
The bounds refer to price levels for the combination of related tradable instruments (spreads) outside which arbitrage activity switches from unprofitable to profitable.
Example:
- 3-cookie package price $p_3$
- 1-cookie price $p_1$
- cost to pay someone to redeem (open) a cookie pack $c_{\textrm{redeem}}$
- cost to pay someone to create (seal up) a cookie pack $c_{\textrm{create}}$.
Arbitrage-free price lower and upper bound price limits for a cookie / package spread:
- profitably buy cookie 3-packs, open package, sell three single cookies when: $$ 3 p_1 -p_3 > c_{\textrm{redeem}}$$
- profitably buy three single cookies, seal up package, sell cookie 3-packs when: $$ p_3 - 3 p_1 > c_{\textrm{create}} $$
- combining these two equations, the arbitrage upper and lower bounds for this example's cookie spread are:
$$ -c_{\textrm{create}} < 3 p_1 - p_3 < c_{\textrm{redeem}}$$
Index arbitrage and ETF arbitrage work in an analogous manner, and the transaction costs yield arbitrage bounds. In general, different market participants have different cost structures, and therefore different arbitrage bounds.