This is the problem:
Let $X=\{a,b\}$ be the topological space with trivial topology and $A=\{a\}$ the one point set (obviously not open). Prove using definition of cofibration that inclusion $A\to X$ is cofibration.
This is the problem:
Let $X=\{a,b\}$ be the topological space with trivial topology and $A=\{a\}$ the one point set (obviously not open). Prove using definition of cofibration that inclusion $A\to X$ is cofibration.