# What number can we give to the aliens?

If the aliens come to ask for this number,
we can give them the answer

I am a pyramid, but not the kind Egyptians made

My rule is relevant to chemistry

What number am I?

• i feel like answer is 42 :p – manshu Feb 25 '16 at 19:19
• @manshu No it isn't. – wythagoras Feb 25 '16 at 19:19
• But 42 is always the answer! – Aggie Kidd Feb 25 '16 at 22:36

You are the number

18.

If the aliens come to ask for this number,
we can give them the answer

The Ramsey number $R(5,5)$ lies between 43 and 49; its exact value is unknown. Erdős asks us to imagine an alien force, vastly more powerful than us, landing on Earth and demanding the value of $R(5, 5)$ or they will destroy our planet. In that case, he claims, we should marshal all our computers and all our mathematicians and attempt to find the value. But suppose, instead, that they ask for $R(6, 6)$. In that case, he believes, we should attempt to destroy the aliens. See also: https://en.wikipedia.org/wiki/Ramsey's_theorem

The Ramsey number $\bf{R(4,4)=18}$ is well-known, and can easily be given to the aliens.

I am a pyramid, but not the kind Egyptians made

18 is a pentagonal pyramidal number; see https://en.wikipedia.org/wiki/Pentagonal_pyramidal_number

My rule is relevant to chemistry

The 18-electron rule is a rule in chemistry used primarily for predicting and rationalizing formulae for stable metal complexes, especially organometallic compounds; see also https://en.wikipedia.org/wiki/18-Electron_rule