Jos Gibbons
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 Nov 17 answered Area of a circle twice to get the volume of a sphere? Nov 8 answered Does $\frac {2 \cdot 4 \cdot 6 \cdot \dots \cdot (2n)} {1 \cdot 3 \cdot 5 \cdot \dots \cdot (2n-1) (n^2)}$ converge? Nov 8 comment Find a cylinder $y=f(x)$ that is tangent to the surface $z^2+2xz+y=0$ at all points common to the two surfaces Completing the square expresses $z$ in terms of $x,\,f$ at the common points of the surfaces. (There are two possibilities that need to be separately explored.) The requirement that the surfaces' normal vectors are orthogonal obtains an expression for $f'$ in terms of $x, z$. This expression may therefore be written in terms of $x, f$. The hard part is solving the differential equations the two cases obtain. Nov 7 comment Non-integrability of the pdf of a squared normal random variable Indeed, the integral obtained by dropping the exponential factor from the integrand provides an upper bound on the cdf, since the exponential factor reduces the integrand in practice. This proves integrability of the pdf. Nov 7 answered Does series with factorials converge/diverge? Nov 7 answered Showing that a set has equal number of even\odd numbered subsets Oct 24 awarded Teacher Jul 16 answered Imaginary numbers and polynomials question Jul 16 answered Solve a trigonometric equation ($\sin(β/2)=β/4$) Jul 16 answered Calculate 2000! (mod 2003) Feb 7 answered Is There A Polynomial That Has Infinitely Many Roots? Jan 11 awarded Student Jan 10 asked Hadamard regularization isn't working out