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The Schwarzschild radius for 11D BHs is given by $l_{11}(l_{11}m)^{1/8}$, which is the special case ($D=11$) of the general dimensional case of $(G_Dm)^{\frac{1}{D-3}}$. Here $m$ is the BH mass and $G_D$ is $D$ dimensional Newton's constant. Now if M-theory is compactified on a torus $T^p$ of sizes $L$s, then I think $G_D$ is related to eleven dimensional Planck length $l_{11}$ as $G_D=\frac{l_{11}^9}{L^p}$. I don't understand how can then the radius smoothly go to the 11 dimensional value if we start decompactifying the torus to get to the 11D case. It seems that it blows up. Where's my mistake? Thanks a lot.

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The formula you have used in general dimensions would only be valid when the Schwarzschild radius (dictated by the mass of the black hole) would be much larger than the compactified dimensions. When you start making L larger to decompactify the torus, the formula starts becoming a worse approximation until it breaks down when the black hole is approximately the size of the compact dimensions.

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