josdejong/mathjs

simplify("(x^2)^0.5") should be "abs(x)"

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#3,329 建立於 2024年11月29日

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 (5 留言) (0 反應) (0 負責人)JavaScript (13,832 star) (1,298 fork)batch import
bughelp wanted

描述

While there are genuine use cases when returning "x" would be better (that's what most humans would expect), strictly speaking it is incorrect.

To reproduce in the mathnotepad:

derivative("(x^2)^0.5", "x").evaluate({x:-3})
    1

derivative("(x^2)^0.5", "x", {simplify:false}).evaluate({x:-3})
    -1

Here I evaluate the same derivative at the same point x=-3 but get two different results. Same problem for x^4 and I assume all other even powers.

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javascript
領域
backendapi
議題類型
bug
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3
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clear
前置要求
JavaScriptbasic algebra
新手友善度1-100 的估計分數，表示該議題對首次貢獻者的友善程度。
70
研究方向
Look at the simplify function in mathjs, particularly the rules for exponent simplification. The issue is that (x^2)^0.5 currently simplifies to x, but should be abs(x) to ensure correctness for negative x. Given that the derivative evaluation gives different results, this is a bug. Check simplify.js and the combsimp logic. Consider adding a rule that converts (x^n)^(1/m) to abs(x)^(n/m) when n is even and m is odd. There are 5 comments that may contain additional context or proposed approaches.