> For the complete documentation index, see [llms.txt](https://dev.spectra.finance/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://dev.spectra.finance/integration-reference/spectra-oracles/deterministic-oracles/linear-apr-model.md).

# Linear APR model

The linear APR model is a pricing model for a Principal Token quoted in underlying. It assumes that the price of the PT is a unit of underlying discounted by a non-compounded implied rate that stays constant during the term. More specifically, is assumes

$$
P(t,T) =\text{PT.previewRedeem(1)}\frac{1}{1+r(T-t)}
$$

where $$\text{previewRedeem(1)}$$ gives the redemption rate of the PT,  $$T$$ is the maturity timestamp, while $$t$$ is the current timestamp.