What do more than or less than sign with a line through them mean?

Question

What do these symbols where the arrow point to mean? Does it mean not less then? If that's the case, why do we write it that way and not > if they are the same?

Answer 1

$0\not<1$ means ‘$0$ is not less than $1$’, just as $a\ne b$ means ‘$a$ is not equal to $b$’. This is not the same as $0>1$; it is the same as $0\ge 1$, but in this case I suspect that $p$ is $0<1$, and the writer preferred to write the negated relation in a way that made the connection with $\sim p$ (‘not $p$’) obvious

Answer 2

Pretty much for any logical symbol in mathematics, a diagonal line through it means the logical negation. Examples $\notin$, $\neq$, $\nexists$, etc.