It is proved that the solution to the initial value problem , u(0,x) = 1/(1+x²), does not belong to the Gevrey class in time for 0 ≤ s < 1. The proof is based on an estimation of a double sum of products of binomial coefficients.
@article{bwmeta1.element.bwnjournal-article-doi-10_4064-cm95-2-9, author = {Grzegorz \L ysik}, title = {Nonanalyticity of solutions to $$\partial$\_{t}u = $\partial$$^2$\_{x}u + u$^2$$ }, journal = {Colloquium Mathematicae}, volume = {96}, year = {2003}, pages = {255-266}, zbl = {1024.35022}, language = {en}, url = {http://dml.mathdoc.fr/item/bwmeta1.element.bwnjournal-article-doi-10_4064-cm95-2-9} }
Grzegorz Łysik. Nonanalyticity of solutions to $∂_{t}u = ∂²_{x}u + u²$ . Colloquium Mathematicae, Tome 96 (2003) pp. 255-266. http://gdmltest.u-ga.fr/item/bwmeta1.element.bwnjournal-article-doi-10_4064-cm95-2-9/