# Topological and Other Strange Captchas

Most Captchas aim to separate humans from spambots, but a few creative Captchas floating on the Web probably aim to separate the super-humans from the mere mortals. Tongue-in-cheek aside, I'd definitely like to see more topological Captchas like the one above.

Geek Technica has a neat list of 10 such Captcas that have gone a bit too far in proving that you're human: Link

Damn, if it isn't C because every single intersection on the fold out has three lines, yet the top left side of C shows a white line crossed only by one other dark green line.

Took me more than 29 seconds though.

ln(2) for the math limit one, because arctan(x) goes to 0 while sin(1/x) can only bounce back and forth widely between -1 and 1. However, the sqrt won't like the parts of the function when sin(1/x) is negative, but if sqrt(-1) is allowed and we make this a complex function you would still get to ln(2) eventually. Probably should expand the functions into Taylor series and then do the limit (maybe use L'Hopital's rule also) to get a more rigorous result. Coincidentally, It also goes to ln(2) when x goes to infinity, this time because sin(1/x) goes to zero.

Braille:
?k_ynn__wa_s__here
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Is it C? There doesn't seem to be a match on the layout to the side of C that is facing us. (there's no cross of a terminating light green and black over a terminating yellow.

I think it might be E. But I'm not sure.
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All kidding aside, these would be bad for me. As I cannot visualize the answer (though one of my sons, at age 5 could easily have done this), I would never succeed.
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