Update C47 lower bound using Aldaz 2000

[sebastian-griego]

Updates the lower bound for C47 using Proposition 1.4 of José M. Aldaz, A remark on the centered n-dimensional Hardy-Littlewood maximal function.

Paper: https://dml.cz/handle/10338.dmlcz/127554
PDF: https://dml.cz/bitstream/handle/10338.dmlcz/127554/CzechMathJ_50-2000-1_14.pdf

Substituting n = 2 in Aldaz's lower bound gives

$$ c_2 \ge \frac{3}{4} - \frac{\sqrt{2}}{4} + \frac{\sqrt{6}}{2} = 1.6211914808\ldots . $$

This improves the currently listed lower bound

$$ \frac{11+\sqrt{61}}{12} = 1.5675208063\ldots . $$