## what does this shell command do? “${f// /_}” [duplicate] This question already has an answer here: • What does the double slash mean in $ {f// /_}? 2 answers

somehow this identifies spaces in filenames and replaces them with underscore, how do I learn how to do this?

## Prove the continuity of $|f|$

Prove that if $$f$$ is continuous at $$x$$ , then so is $$|f|$$.

## If $f$ is uniformly continuous on $[a,\infty)$,and $f’$ has definition on $[a,\infty)$,can one deduce that $f’$ is bounded?

If $$f$$ is uniformly continuous on $$[a,\infty)$$,and $$f’$$ has definition on $$[a,\infty)$$,can one deduce that $$f’$$ is bounded on $$[a,\infty)$$?

I know some functions like $$\sqrt{x}$$ which has definition on the open set $$(0,\infty)$$ , but which derivative is not bounded, so I wish the boundedness will be hold on a closed set $$[a,\infty)$$

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## finding the extreme values of $f$.

If $$f(x,y) = x ^2 + xy + y^2 – 4 \ln x – 10 \ln y,$$ and I found $$f_{x}^{‘} = 2x + y – 4/x$$ which means, $$2x^2 + xy = 4………….1.$$ and also I found $$f_{y}^{‘} = 2y + x – 10/y$$ which means, $$2y^2 + xy = 10 ……> 2.$$

then I subtracted 1. from 2.and I got $$y^2 – x^2 = 3,$$

But then I do not know how to complete, can anyone help me please?