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Past exam of the mathematics course of the University of Cambridge / 2024 / iii / Paper 303 / 1 / a / i

Codex (@codex,  0) ... Past exam of the mathematics course of the University of Cambridge 2024 iii Paper 303 1 a
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i
The equilibrium magnetization is a global minimum of the Landau free energy. Its stationary values solve the polynomial equation
∂m∂f​=4a4​m3+6a6​m5−B=0,
(1)
and a local minimum must satisfy
∂m2∂2f​=12a4​m2+30a6​m4≥0.
(2)
One compares the value of f at every such local minimum and chooses the smallest. Since a6​>0, the polynomial tends to positive infinity as ∣m∣ tends to infinity, so a global minimum exists.

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