- What is the meaning of the expression Q. E. D. ? Is it similar to . . .
Putting QED after that sentence was their way of saying, "and that's what I was trying to prove, so there, I've proved it" We aren't so strict about proofs anymore, but as a practical matter it's very useful to have a definitive mark that indicates the proof is over Some authors use Q E D as that mark, some authors use the box
- What are the advantages of ending a proof with “QED”?
QED is short for the Latin phrase quod erat demonstrandum and means, loosely, "that which was to be shown" As noted in the comments, it's a summary way of saying that the proof is concluded
- 量子电动力学（QED）有多精确？ - 知乎
量子电动力学（QED，Quantum Electrodynamics） 是描述电磁相互作用的理论，可以说是目前为止最为精确的理论了。 在量子力学提出之后（主要标志为1926年薛定谔提出薛定谔方程），狄拉克紧接着于1927年提出了狄拉克方程用于描述电子与光子的相互作用。
- 以「Q. E. D」收尾证明语段，是什么梗？ - 知乎
(also written QED and QED) is an initialism of the Latin phrase quod erat demonstrandum, meaning "what was to be demonstrated", or, less formally, "thus it has been demonstrated"
- Why does drawing $\\square$ mean the end of a proof?
I had always used QED (quod erat demonstrandum) at the end of a proof until I was introduced to using Halmos' symbol Since it was a 4-sided symbol, my mind has always associated this "quad" symbol with "quod"
- 在数学高考证明题中写下Q. E. D. 可能被判标记卷吗？ - 知乎
众所周知 Q E D 是quite easily done的缩写 挥挥洒洒地终结证明，不经意间注记quite easily done，大佬的高考，往往就是这么朴实无华，且枯燥 而阅卷老师只会在佩服之余，点下满分按钮
- Usage of the term Q. E. F. - Mathematics Stack Exchange
It's just a phrase, so it should be used where appropriate One "makes" or "does" a construction, and "proves" or "demonstrates" a theorem So, QEF when a construction is finished, and QED when a proof is finished
- Is $\\square$ an alias for QED? - Mathematics Stack Exchange
In math, proofs are often accompanied by 'QED' (quod erat demonstrandum) at the end, indicating their conclusion Some authors use a square symbol ($\\square$) instead of QED, with the same meaning
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