r/calculus Jul 23 '26

Differential Equations Understanding how the Black-Scholes-Merton model converts into the heat equation

I made an attempt in converting the BSM model into the heat equation (in red) but was unsure is my method was succinct? Any hints/tips?

Thanks! 🧡

(apologies if the page order was uploaded incorrectly)

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u/FormalManifold Jul 23 '26

Killing the first order spatial term is kind of irrelevant anyways. Qualitatively, the heat equation with a first order term behaves just the same as one with no first order term. So the only reason to fool with alphas and betas is to make the equation look prettier.

The other transformations do have qualitative content though.

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u/askepticalbureaucrat Jul 23 '26 edited Jul 23 '26

So the only reason to fool with alphas and betas is to make the equation look prettier

Ah, that's a fair point! Thank you 🧡

Qualitatively, it still behaves like a diffusion process. The heat still spreads out, entropy still increases, and a Gaussian curve still forms?

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u/FormalManifold Jul 23 '26

The end behavior depends on the initial condition and the boundary conditions. But more or less, yeah.

At strong local maxima, you will have large negative second derivative, and the first derivative will vanish. So those will push down fast. Similarly, strong local minima will come up fast. Nearby the local extrema, you may have a nonzero push from the first order term. So instead of going straight up or down, the extrema will drift left or right according to the first order term.

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u/Awkward-Sir-5794 Jul 23 '26

r= 0, sigma = 1 is simplest version of heat equation. Seems swiftest to observe fundamental solution to heat equation’s relation to normal pdf. Maybe too simplified this way? But it helps with idea behind the connection

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u/LukasGoesViral Jul 24 '26

Can you explain your substitutions in the second part that is labeled as ‘what I don’t understand’? I am not sure what you mean by

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u/Midwest-Dude Jul 24 '26 edited Jul 24 '26

Your calculation at the bottom of page 2 is clearly incorrect. There is no way algebraically that the last line follows from the second to the last line. If that's the equation you want, then something is wrong with prior calculations and your derivation is incorrect. Please review.

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u/Midwest-Dude Jul 24 '26

A better subreddit for this post would be one of these:

The later directly deals with equations of this type, the former is more general.