Even for quadratic number fields, finding class numbers is still quite hard.
即便是在二次数域上，求解类数还是相当困难的。
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Generally, this problem is reduced to quadratic residue problem of modulo a big prime number. But this reduction is not applicable to Optimal Extension Fields (OEF).
一般而言，该问题归结为模大素数的二次剩余问题，但这种归结不能用于最优扩域OEF。
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