So I have an expression, let's call it f11 which is a function of t of this form f11= a1*t^(-4)+ a3*t^(-3)+... and so on up until a finite positive power of t. In the expression there are some coefficient of the powers of t, for example \[Sigma]1. Now if I write something like this:
f11= bla bla
\[Sigma]1 = 0;
Series[f11, {t, 0, -2}]
these 3 errors appear
- Power::infy: Infinite expression 1/0^2 encountered.
- Power::infy: Infinite expression 1/0 encountered.
- Infinity::indet: Indeterminate expression ComplexInfinity+ComplexInfinity encountered.
If I instead write
f11= bla bla
Series[f11, {t, 0, -2}] /. \[Sigma]1 -> 0
no error occours and everything is fine, why is that so?
I'd prefer to set the variable \[Sigma]1 to 0 once and for all not apply the rule /. \[Sigma]1 -> 0 everytime so having to apply the rule is kind of a nuisance
EDIT: Also I want to point out that I have other parameters inside f11 like \[Sigma]2 or a1 and so on and when I assign them to 0 they cause no problem whatsoever, but thye enter inside f11 more or less in the same way \[Sigma]1 does. In fact in the rest of the notebook, I assign \[Sigma]2=0 and then go on writing the series expansion of f11 but I always have to keep the rule /. \[Sigma]1 -> 0
EDIT 2: To make it clearer I will post here the essentials of my notebook:
a = A*(t + a2*t^2 + a3*t^3 + a4*t^4 + O[t]^5);
H = (\!\(
\*SubscriptBox[\(\[PartialD]\), \(t\)]a\))/a;
\[Sigma] = \[Sigma]i*(1 + \[Sigma]1*t + \[Sigma]2*t^2 + \[Sigma]3*
t^3 + \[Sigma]4*t^4 + O[t]^5);
f1 = H^2 ==
a^2*(\[Rho]r0/a^4(*+(\[Rho]m0/a^3)*)+ V)/(
3*\[Gamma]*\[Sigma]^2) + (\!\(
\*SubscriptBox[\(\[PartialD]\), \(t\)]\[Sigma]\))^2/(
6*\[Gamma]*\[Sigma]^2) - 2*H*\!\(
\*SubscriptBox[\(\[PartialD]\), \(t\)]\[Sigma]\)/\[Sigma] + (\!\(
\*SubscriptBox[\(\[PartialD]\), \(t\)]\[Sigma]\))^3/(
3*\[Gamma]*a^2*\[Sigma]^2)*(6*g*H - dg*\!\(
\*SubscriptBox[\(\[PartialD]\), \(t\)]\[Sigma]\));
f11 = Collect[Expand[f1], t];
At this point I do the commands mentioned above and I have the problem mentioned above. I do it in this way cause f11 is a bit of a long expression and I want to isolate the term order by order and see them, without necessarly know them in advance. For clarity I will also post the screenshots from my notebook which would be easier to read with respect to the code written in that way
Some quantities that enter f11:
Definition of f11 and errors with assignment [Sigma]1 = 0:
Definition of f11 and no errors with rule /.[Sigma]1->0]:
Complete expression of f11:
