From a colleague’s letter:

Here the teaching has finished and the exam period has started. I had some midterms couple of weeks ago in which a student was trying to prove \(\sqrt{2}\) is irrational. they defined a rational number \(\frac{a}{b}\) with \(b=0\).

I think at that point mathematics upped and left the country. I had another who had made some calculation mistake in the first step of a proof by induction, and ended up with \(8>9\). They dutifully then marked it as \(P(1)\) holds and continued.

I had others that started with \(8 \mid 5^{2n} -1\) (\(n>1\) ) and put it “equal” to many things and ended up with

\[

\dots = \frac{8}{5^{2n} -1 } = 3

\]

At this point I think logic decided to follow maths out of the country …

What can I say?

Keep calm and carry on.

Wikipedia says about the now famous poster:

“Printing began on 23 August 1939, the day that Nazi Germany and the USSR signed the Molotov–Ribbentrop Pact, and the posters were ready to be placed up within 24 hours of the outbreak of war. Almost 2,500,000 copies of Keep Calm and Carry On were printed between 23 August 1939 and 3 September 1939 but the poster was not sanctioned for immediate public display. “

Apparently the Blitz was not judged to be desperate enough situation — I personally see a great moral lesson in that, especially for us, mathematics teachers.