Timeline for Should the formula for the inverse of a 2x2 matrix be obvious?
Current License: CC BY-SA 4.0
34 events
| when toggle format | what | by | license | comment | |
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| Mar 7 at 0:41 | answer | added | Michael Hardy | timeline score: 0 | |
| Mar 6 at 23:58 | answer | added | Dan Piponi | timeline score: 8 | |
| Apr 16, 2023 at 6:55 | review | Close votes | |||
| Apr 16, 2023 at 19:45 | |||||
| Apr 16, 2023 at 1:56 | answer | added | Christopher Gallegos | timeline score: 4 | |
| Feb 21, 2021 at 9:41 | review | Close votes | |||
| Feb 22, 2021 at 12:19 | |||||
| Feb 21, 2021 at 2:59 | answer | added | Venkata Karthik Bandaru | timeline score: 0 | |
| Aug 14, 2020 at 15:47 | history | edited | LSpice | CC BY-SA 4.0 |
Tidying the post while it's on the front page anyway
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| S Dec 12, 2017 at 1:13 | history | suggested | Rodrigo de Azevedo |
Added 2 tags
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| Dec 11, 2017 at 20:45 | review | Suggested edits | |||
| S Dec 12, 2017 at 1:13 | |||||
| May 4, 2017 at 15:50 | comment | added | user21230 | $4371$=$47*31*3$ is the dimension of Baby Monster representation. | |
| Mar 20, 2017 at 12:53 | answer | added | user21230 | timeline score: 5 | |
| Jun 19, 2016 at 23:48 | comment | added | tvk | I remember it by thinking $I^{-1} = I$ (which one won't ever forget), so you can only make the off-diagonals negative. | |
| Apr 30, 2015 at 13:37 | comment | added | Gerry Myerson | I'd feel much better about this question if only it said somewhere, "assuming $ad-bc\ne0$." | |
| Apr 27, 2015 at 4:17 | comment | added | Tom Copeland | The geometry associated with Poloni's answer makes the algebra transparent, is easy to remember, and is suitable for introductory lessons on matrices and vectors in linear algebra. | |
| Dec 29, 2013 at 18:05 | comment | added | Noam D. Elkies | (These look like fine memory aids, but you must mean "mnemonic", not "pneumonic"...) | |
| Sep 28, 2013 at 13:15 | comment | added | Hiro Lee Tanaka | There is a pneumonic I teach my students: "NBC is sad." For some reason the television station and the emotion are memorable. It stands for "negate B and C, switch A and D." This isn't geometric intuition, but I've never forgotten the pneumonic! Another pneumonic I like is "MARBLE", which is not linear algebra. It stands for "Maps at Right Blank are Left Exact." (Because I always forget the convention for left and right exact functors.) | |
| Feb 23, 2012 at 8:35 | answer | added | Federico Poloni | timeline score: 26 | |
| Feb 22, 2012 at 15:33 | answer | added | Grant Lakeland | timeline score: 27 | |
| Feb 22, 2012 at 13:31 | answer | added | Martin Brandenburg | timeline score: 14 | |
| Feb 21, 2012 at 15:07 | vote | accept | Frank Thorne | ||
| Feb 21, 2012 at 7:39 | comment | added | Jonny Evans | You know you want to get the determinant along the diagonal, which is ad-bc, so you know the first column has to be d,-c. Likewise for the second column. | |
| Feb 21, 2012 at 7:31 | answer | added | Tobias Hagge | timeline score: 5 | |
| Feb 21, 2012 at 4:57 | answer | added | Scott Carter | timeline score: 12 | |
| Feb 21, 2012 at 4:38 | answer | added | Noam D. Elkies | timeline score: 258 | |
| Feb 21, 2012 at 4:02 | answer | added | Qiaochu Yuan | timeline score: 45 | |
| Feb 21, 2012 at 3:27 | comment | added | Will Sawin | Look at it mod $20$. $3\times 7=1$. | |
| Feb 21, 2012 at 3:23 | comment | added | Suvrit | I remember it in a boring form: the diagonals are easy, so they just change places, while the off-diagonals are special, so they suffer a sign "inversion" -- actually, hardly anything to remember ;-) | |
| Feb 21, 2012 at 2:58 | answer | added | Daniel Litt | timeline score: 83 | |
| Feb 21, 2012 at 2:40 | comment | added | Daniel Litt | I was just discussing this with a friend; I think this is a great pedagogical question. | |
| Feb 21, 2012 at 2:34 | history | edited | Frank Thorne | CC BY-SA 3.0 |
deleted 220 characters in body
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| Feb 21, 2012 at 2:30 | history | edited | Ori Gurel-Gurevich | CC BY-SA 3.0 |
deleted 35 characters in body
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| Feb 21, 2012 at 2:30 | comment | added | Benjamin Steinberg | Frank: enclose paragraphs with matrices in <p>...</p> | |
| Feb 21, 2012 at 2:28 | history | edited | Benjamin Steinberg | CC BY-SA 3.0 |
added 21 characters in body
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| Feb 21, 2012 at 2:09 | history | asked | Frank Thorne | CC BY-SA 3.0 |