Question Number 128105 by snipers237 last updated on 04/Jan/21
$${Let}\:{f}\:\:\:\mathbb{N}^{\ast} \rightarrow\mathbb{N}^{\ast} \:{a}\:{bijection}\: \\ $$$${Prove}\:{that}\:\:\underset{{k}=\mathrm{1}} {\overset{{n}} {\sum}}{f}\left({k}\right)\geqslant\mathrm{1}+\mathrm{2}+\mathrm{3}+...+{n} \\ $$