# How can I evaluate a symbolic expression as a transfer function

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Robert Ungi on 8 Jun 2015
Answered: Walter Roberson on 8 Jun 2015
Hi,
I have the following transfer function G(s) = 1/(s^2+1)
I have the following symbolic expression
syms G_s_p G_s_m
sys = [ G_s_m/2 + G_s_p/2, (G_s_m*i)/2 - (G_s_p*i)/2;
- (G_s_m*i)/2 + (G_s_p*i)/2, G_s_m/2 + G_s_p/2]
where G_s_p = G(s+i*w_0) and G_s_n = G(s-i*w_0)
How can I create transfer function from the sys variable?
Thank you, Robert

Walter Roberson on 8 Jun 2015
I do not know my way around the various representations of transfer functions. In part it is going to depend upon what output form you want: if you want something that can be used by the control system toolbox then you will need to convert to numeric form. But there is also symbolic work with transfer functions such as http://www.mathworks.com/help/symbolic/ztrans.html

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