Proving $x^3$ is not convex.
135867
$f(x)=x^3$ is convex nor concave without to derivate
138457
Seminorms: Absolute Convexity
88645
Prove that the additive groups $\mathbb{R}$ and $\mathbb{Q}$ are not isomorphic.
116851
Show that f(x) is convex
11259
Prove $R\times S$ is not an integral domain
6335
Proving that the set enclosed by an ellipse is convex
12189
Prove norm doesn't come from inner product.
67313
Proving $\{(x_1, x_2) \in \mathbb{R}^2_{+} \mid x_1 x_2 \geq 1\}$ is convex
143999
Prove that formula is not tautology.
35953
Prove that a function is not injective
58009
How to show that $\mathbb Q(\sqrt 2)$ is not field isomorphic to $\mathbb Q(\sqrt 3).$
156457
Show that $K[x,xy,xy^2,\dots]$ is not Noetherian
27933
Show that unit circle is not homeomorphic to the real line
116382
Showing convexity of a function with the restriction over an arbitrary line
83023
From axiom of regularity, how can I prove $x \notin y \lor y \notin x$
85819
A non-polyhedral pair $(X,A)$ with both $X$ and $A$ polyhedral
125175
convexity proof of a function including ln and sums