Quotes surely. Cryptogram #1

Question

Cryptogram #1

Iq iycsc mqks gs wgqskowi gcieccq qqwocqi qqm agmccq siymocs, iyccc aysi sycckk gc sgaciyoqi ig gc sqom sgc q siymk eyowy mom qgi gcioq eoiy Pkiyqigcqs, qqm eokk qgi cqm eoiy Eoqsicoq, gyi os iyc gkmcsi qqm iyc kgyqicsi gs qkk.

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Hint:

$${\mathfrak {\frac Em=a^2+b^2}}$$

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Answers must include method of decryption and the thinking that led to do certain things to solve the puzzle along with plain text.

Answer 1

To complete the answer by TheEssential, the encryption method is:

Upper case letters remain unchanged.

Lower case letters are encrypted as shown in the table below. Two plaintext letters are encoded to one ciphertext letter.
The initial CT character is "q" which is "a" plus 16 (16 is the sum of digits of 15262 in the PSE question number of the FQDN). Then, the sequence of ciphertext letters is -11, +15, -11, +15... applying mod-26.

Answer 2

I believe the quote is:

In these days of conflict between ancient and modern studies, there must surely be something to be said for a study which did not begin with Pythagoras and will not end with Einstein, but is the oldest and youngest of all.

Your hint admittedly makes it pretty simple: $a^2 + b^2 = c^2$ and $\frac {E} {m} = c^2$ are of course, the two most famous equations to come out of Pythagoras and Einstein. Pkiyqigcqs becomes Pythagoras and Eoqsicoq becomes Einstein, and you can map some known letters as so:

a       f       k - y p       u     z
b       g - o   l     q - n/a v
c - e/r h       m     r       w
d       i - t/g n     s - s   x
e       j       o - i t       y - h

Each of the pairs you see are just a ROT13 pair, so any letters that you find, you can also find their equivalent. That further develops the tableau. I think the way to fill out the rest of it slipped me by due to personally recognizing the quote.