# LogisticSigmoid residue bug?

Bug introduced in 10.0 and fixed in 10.2.0

On version 10.0 for arm:

  Residue[LogisticSigmoid[z],{z,I Pi}]


gives 0 not 1.

Can anyone reproduce this?

• As noted in its tag wiki, please do not give questions the bugs tag until your results have been community-vetted as an actual bug. – J. M. will be back soon May 21 '15 at 12:14
• Anyway, for diagnostics: what happens if you feed Residue[] the result of FunctionExpand[LogisticSigmoid[z]]? – J. M. will be back soon May 21 '15 at 12:17
• I get 1, which is correct. – pdmclean May 21 '15 at 12:52
• Can confirm in 10.1, Residue[LogisticSigmoid[z],{z,I Pi}] gives 0 while Residue[FunctionExpand[LogisticSigmoid[z]], {z, I Pi}] gives 1. What's going on? – Histograms May 21 '15 at 14:59
• Reported as a bug internally. Thanks! – ilian May 21 '15 at 15:16

Now fixed in version 10.2.

In[1]:= Residue[LogisticSigmoid[z], {z, I Pi}]

Out[1]= 1

In[2]:= Residue[1/(1 + Exp[-z]), {z, I Pi}]

Out[2]= 1

In[3]:= Coefficient[Series[1/(1 + Exp[-z]), {z, I Pi, 0}], 1/(z - I Pi)]

Out[3]= 1

• ilian, if I haven't already done so: thanks for all the work you and your team put into this program we all like. :) – J. M. will be back soon Jul 14 '15 at 17:34
• @J. M. Thank you very much for the kind words! – ilian Jul 14 '15 at 18:01