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I want to subtitute $x = 1$ into the expression $$f(x)=x^2 + 3 (x^2 - m^2) + \sqrt{x^2 + 1}$$ but I don't calculate the value $f(1)$, that is mean, I want $$1^2 + 3(1^2-m^2)+\sqrt{1^2 + 1}.$$ If I tried 

f[x_] := x^2 + 3 (x^2 - m^2) + Sqrt[x^2 + 1]; f[1]

I receive

1 + Sqrt[2] + 3 (1 - m^2)

How do I tell Mathematica to do that?

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  • $\begingroup$ Dirty trick : f["1"]. $\endgroup$ Commented Feb 26, 2013 at 8:21

1 Answer 1

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$\begingroup$ 
 f[x_] := x^2 + 3 (x^2 - m^2) + Sqrt[x^2 + 1]; f[HoldForm@1] (* thanks: @chris *)

or

 f[Defer@1]

enter image description here

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  • 2
    $\begingroup$ what's wrong with f[Defer[x]] /. x -> 1 ? The order? $\endgroup$ Commented Feb 26, 2013 at 8:38

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