Perfect Liquidity as an Idealized Market Modeling Assumption

Summary

The document explains perfect liquidity as a simplifying assumption in mathematical models of financial markets. Under this idealization, a model abstracts away practical trading restrictions, allowing the analysis to focus on the joint evolution of asset prices. Examples of restrictions that the assumption ignores include prohibitions on short selling and limits on the volume that can be traded. It also treats trading arbitrary quantities as possible, even when real markets permit only discrete units or impose other constraints.

This assumption does not remove restrictions from actual markets; it omits them from the model to make the mathematical setup easier to handle. The answer is a brief conceptual clarification rather than a formal liquidity model, empirical study, or account of transaction costs and price impact. As a result, conclusions derived under perfect liquidity may not account for implementation limits that matter in practice.

Key ideas

• Perfect liquidity lets a market model abstract away trading restrictions.
• The idealization can permit short selling and trading at unrestricted volumes.
• It may treat arbitrary quantities as tradable even when real markets impose discrete units.
• The assumption simplifies mathematical analysis but does not describe practical market constraints.

Tags

# Assuming perfect liquidity


# Assuming perfect liquidity












I encountered this phrase in the textbook by Hans Schumacher

> For the purposes of this textbook, a mathematical model for a financial market consists of a specification of the joint evolution of prices of a number of given assets. Further information that may be important in practice, such as trading restrictions, are abstracted away by the assumption of perfect liquidity.

Could anyone explain how assuming perfect liquidity would resolve issues like trading restrictions?

## Answer by Frido (score 5, accepted)

https://quant.stackexchange.com/a/75597

A trading restriction could mean that you cannot short certain instruments, or that you cannot execute above/below certain volumes. For example in practice you cannot trade fractional numbers of a stock, let alone irrational numbers. Perfect liquidity basically means none of these restrictions exist, it is an idealization to facilitate modelling.

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This summary was written by Stratmill's research agent from the original; it is not a copy of the source.