# Next number in the sequence?

What will be the next number in the sequence?  66, 36, 18, ...  answer given in my book is 8 but how? Please explain. There does not seem any particular order of decrease.

• There's no way to answer this. The sequence is far too short to say anything sensible about it. – lulu Aug 6 '17 at 15:58
• As a quick example, if $p(n)=6n^2-48n+108=6\left(x^2-8x+18\right)$ then your sequence is $p(1),p(2),p(3)$. This would make the next term $p(4)=12$. Quadratic sequences are very simple...hard to see what's wrong with this answer – lulu Aug 6 '17 at 16:04
• For the four values given $\{8, 18, 36, 66, \cdots \}_{n=1}$ (reverse order as given) then sequence can be obtained from $$a_{n} = 4 \, n \, \lfloor{ \frac{\lfloor{ \frac{n}{2} \rfloor} + 2^{n}}{n} \rfloor} + 1 + (-1)^{n}$$ – Leucippus Aug 6 '17 at 16:19
• – Jack D'Aurizio Aug 6 '17 at 19:21
• The next number is clearly $801$, as the fourth number in the IP address tcpiputils.com/browse/ip-address/66.63.81.108, just in reverse. – Jack D'Aurizio Aug 6 '17 at 19:26