# Tagged Questions

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### MATLABs double arithmetic

this is a classical problem, but I need help to pinpoint what I am missing. Problem: In MATLAB (exp(1) + 10^12) - 10^12 gives you a double which equal to e, up to 5 correct digits. But I thought ...
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### Implementing std::nextafter: Should denormals-are-zero mode affect it? If so, how?

This might be the wrong stackexchange site for this question. math.SE, cs.SE, programmers.SE, and of course stackoverflow are all possibilities. I'm hoping to reach an audience that might actually ...
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### Divide and Conquer division algorithm explained (as used in GMP bignum)

I am trying to understand the divide and conquer division algorithm that is used in the GMP bignum arithmetic library. The code is very optimised and that makes it somewhat hard to understand. the ...
84 views

### How to construct a subring of a polynomial ring in Magma

This is a question about the computer algebra system Magma. I have been looking for a place to ask this type of question on the SE network and scicomp.SE was suggested to me; hopefully it finds a home ...
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### How can I avoid roundoff error when calculating the difference $\textrm{erfc}(a) - \textrm{erfc}(b)$?

In this excellent answer, it is recommended that one make use of the $\textrm{erfcx}$ function to avoid roundoff error in calculating dealing with $x < 25$ (approximately). So, one scales their ...
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### How would I figure out when a function would be at the “brink of underflow”?

User hardmath, provided an excellent overview of how to handle overflow when calculating the product of two functions, where one is likely to overflow: http://scicomp.stackexchange.com/a/20913/9466 ...
260 views

### what does -ffast-math do?

What kind of optimisations does the option -ffast-math do ? I saw that the time taken for a simple $O(n^2)$ algorithm being reduced to that of an $O(n)$ algorithm ...
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### performance of icc main.cpp == g++ -ffast-math main.cpp

I have a program that has a nested loop, together with its parent running at $O(n^2)$ complexity performing floating point arithmetic. I see that the performance of the code when compiled with ...
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### Exact analytical matrix inversion of sparse 100x100 matrices in C++

I need to invert a matrix. Of course, I'm not the first person in this situation, and I know that there's a wealth of powerful libraries out there, of which I only know a couple. That being said, ...