formula of cutoff frequency

Hi,

I want to know if the formula used for calculation of rectangular waveguide cuttoff frequency applicable to circular waveguide?

formula for cutoff frequency

F=c/2a

c is velocity of light
a is dimensions of waveguide

Another formula is

F=(1/2(με)?)((m/a)2+(n/b)2)?

where μ is permeability constant and ε is dielectric constant.

where m,n are modes of waveguide and,

a,b are dimensions of rectangular horn.

Please guide if this formula can be used for circular waveguide?

otherwise kindly provide desired formula with reference of text.

Thanks.