Can I Square Root A Series?
I have a question in Quantum Mechanics where I need to solve a series, and the thing is that I can get the answer to a similar series with the help of the same problem but I am not sure if I can square root my series to use it in the problem. For example I have $$\sum_{n=1,3,5,7...}^{\infty} \frac{1}{n^4} = \frac{\pi^4}{96}$$. But the summation I need is for $\style{Bold}{\frac{1}{n^2}}$ So is it fine to square root both sides and say $$\sum_{n=1,3,5,7...}^{\infty} \frac{1}{n^2} = \frac{\pi^2}{\sqrt{96}}$$
No. For the same reason that $$ \sqrt{a_1+...+a_n}\ne \sqrt{a_1}+....+\sqrt{a_n} $$ What you are saying would also imply that say the harmonic series converges, which it does not.
You are asking if $\sqrt{(\sum a_i)} = \sum(\sqrt {a_i}) $.
Does it? Does $5=\sqrt {9 +16} = \sqrt {9} + \sqrt {16}=7 $?
Less rude, more helpy:
There is the Schwarz inequality:
$(\sum ab)^2 \le \sum a^2 * \sum b^2$
So $\sum 1/n^4 = \pi^4/96$ obviously does not mean $\sum 1/n^2 = \pi^2/\sqrt{96}$ but it does mean $\sum 1/n^2 \ge \pi^2/\sqrt{96}$.
It further means $\sum 1/n \ge \pi/\sqrt[4]{96}$ which it is.
$\sqrt{a} + \sqrt{b} \ge \sqrt{a + b}$ (hence $7 = \sqrt{9} + \sqrt{16} \ge \sqrt{9+16} = 5$.
That can be useful.