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  1. Prikvačeni tweet
    2. lip 2019.

    Something that is very difficult to find anywhere else on the internet -- a clear and precise geometric definition of a limaçon, with explanations of the meaning of each parameter and a description of a polar coordinate system so that one may prove the well-known formula.

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  2. prije 2 sata

    Homemade 3D-printed and hand-painted snub cube earrings by !!

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  3. proslijedio/la je Tweet
    prije 16 sati
    Odgovor korisnicima i sljedećem broju korisnika:

    Yep, that's a really good point. One of the reasons to *choose* a particular codomain is to actually do mathematics on the complement of the image. (Which, as you say, is the entirety of the topology & geometry of knots!)

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  4. proslijedio/la je Tweet
    2. velj
    Odgovor korisniku/ci

    The time when it should come up, but doesn’t, is during the construction of inverse functions. For f: A→B, modify it to make f’: A’→range(f) (A’⊆A) so that f’ is a bijection. Then f⁻¹ is just f’ with domain and codomain swapped and order within each ordered pair reversed.

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  5. proslijedio/la je Tweet
    2. velj
    Odgovor korisnicima

    when i’m reading other people’s work, i interpret equality of functions being defined by having equal domains and codomains, and the images of their domains being equal. this is the only way we could have a notion of a function being intrinsically surjective.

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  6. proslijedio/la je Tweet
    4. kol 2019.
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  7. 2. velj

    Are f: ℝ→ℝ f(x)= x² and g: ℝ→[0,∞) g(x)= x² the same function (only the ordered pairs matter; codomains are not real) or different functions (sorry, but this is how it has to be; non-surjections are a thing)

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  8. 2. velj
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  9. proslijedio/la je Tweet
    30. sij

    Dividing a quadrilateral into two equal areas

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  10. proslijedio/la je Tweet
    31. sij

    Thanks to an email from a reader, I found a compass and straightedge "angle trisection" online. Spoiler alert: it doesn't. But, I was amazed by how close it was to an actual trisection. The person's construction was pretty complicated, but I distilled it

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  11. proslijedio/la je Tweet
    30. sij

    one of the best parts of the impeachment process is watching people be like "Republicans think you can win a trial by lying and blocking relevant evidence!" like yes you absolutely can do that, literally every trial is like this, there is no justice and there is no god

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  12. proslijedio/la je Tweet

    この機構のある1つの点はアステロイドを描いています アステロイドの媒介変数表示は x=cos(θ)^3 y=sin(θ)^3 これでなんとなく仕組みがわかった方もいらっしゃるかも

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  13. proslijedio/la je Tweet
    29. sij

    Dodacedra growing until they fill the hyperbolic space. Each has an inscribed cube. While these cubes do not fill the hyperbolic space (only 40%), the rest can be filled with the same cubes (12-hedron = cube + 6 quartercubes). (Observed by MagmaMcFry in the HyperRogue discord.)

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  14. 29. sij

    It may just not even be possible to clearly state what the range of a function is. Take some 8th degree polynomial which has an absolute maximum at x= (the solution of some 7th degree equation, not expressible in terms of radicals...)

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  15. proslijedio/la je Tweet
    29. sij
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  16. proslijedio/la je Tweet
    28. sij

    (Not just any subset, but that's not the important bit here.) Crucial to this is that before I can start talking about the function itself, I *have* to know what A and B are. In other words, a function begins with its domain and codomain and proceeds from there. 13/n

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  17. proslijedio/la je Tweet
    28. sij

    As you keep zooming in, you think: it's a point! No, it's a line! No, it's a rectangle! No, it's a bunch of points! No... It's a subset of the plane invented by Simon Willerton, which shows that your estimate of the dimension can keep changing as you zoom in more and more.

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  18. proslijedio/la je Tweet
    28. sij

    Before you welcome analytic philosophy into your life think back to that drummer friend in high school who wouldn't stop bippity-bapping on everything and ask yourself if you want that but with first-order logic

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  19. proslijedio/la je Tweet
    27. sij
    Odgovor korisniku/ci
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  20. proslijedio/la je Tweet
    26. sij

    323 years ago , Johann Bernoulli challenged Isaac Newton with a six month deadline to solve the problem of the brachistochrone: Newton got the message on January 29 at 4 pm and solved it going to bed that same night [more: ]

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  21. proslijedio/la je Tweet
    26. sij

    This is a recently discovered illusion, and it’s really quite striking. The strange effect is called the ‘curvature blindness’ illusion

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