How to add an asterisk symbol in a mathematical expression?

问题

I'm trying to annotate a plot with R² value and significance coding, but I can't pass * as a symbol and not as the juxtaposition operator.

I've tried ?plot.math, here is what I tried

plot(1:10,1:10)
text(6,4,expression(R^2==8))
text(6,4,expression(R^2==8^{**}))
Error: unexpected '^' in "text(6,4,expression(R^2==8^{**"

回答1:

You need to use paste inside your expression:

text(4,6,expression(paste(R^2==8^"**")))

or

text(6,4,expression(paste(R^2==8, "**")))

来源：https://stackoverflow.com/questions/14871706/how-to-add-an-asterisk-symbol-in-a-mathematical-expression