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In my Mathematica 11.3.

MatchQ[-3*i[1], -x_*i[1]]

returns False. But

MatchQ[-3*i[1], x_*i[1]]

returns True. I don't understand the reason. Does the minus "-" do something bad?

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  • $\begingroup$ Yes, the Minus is the issue: Head[-3] is Integer, not Minus. But Head[-x] is Minus, so also MatchQ[-3, -x_] returns False. $\endgroup$ – Henrik Schumacher May 1 '18 at 13:30
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    $\begingroup$ In a situation like this the first thing to do is check the FullForm of the expression. $\endgroup$ – Daniel Lichtblau May 1 '18 at 13:43
  • $\begingroup$ fyi, if you want to match negative coefficients you may try MatchQ[-3*i[1], x_*i[1] /; x < 0] (returns true) $\endgroup$ – egwene sedai May 1 '18 at 16:25

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