# Issue in D[ ] in 12.3.0 but not in 12.1.0

Posted 6 months ago
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 Here is the terminal log from 12.1.0, where everything is correct: $math12 Mathematica 12.1.0 Kernel for Linux x86 (64-bit) Copyright 1988-2020 Wolfram Research, Inc. In[1]:=$Assumptions = Element[x,Reals] Out[1]= x \[Element] Reals In[2]:= a=Exp[x] x Out[2]= E In[3]:= D[a*Conjugate[a],x] // ComplexExpand // Simplify 2 x Out[3]= 2 E and here is the log from 12.3.0 which shows the problem: $math Mathematica 12.3.0 Kernel for Linux x86 (64-bit) Copyright 1988-2021 Wolfram Research, Inc. In[1]:=$Assumptions = Element[x,Reals] Out[1]= x \[Element] Reals In[2]:= a=Exp[x] x Out[2]= E In[3]:= D[a*Conjugate[a],x] // ComplexExpand // Simplify 2 x Out[3]= E (1 + I Im[Conjugate'[x]] + Re[Conjugate'[x]]) In[4]:= D[a*Conjugate[a],x] // ComplexExpand[#, TargetFunctions->Conjugate] & // Simplify 2 x Out[4]= E (1 + Conjugate'[x]) Of course, if I did "ComplexExpand" and "Simplify" prior to differentiation, then it would work in 12.3.0 also, but this is clearly a bug, i.e. differentiation should be aware of the assumption that 'x' is real-valued. And in 12.1.0 it was aware, as the log above shows.
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Posted 5 months ago
 Thank you very much for the replies.