Hi Victor,
> > ‘What number is the odd one out, and why: 2 4 6 8 13 19.’
>
> 2 is the only even prime.
> 4 is the only square in the list.
> 6 is the only perfect number in the list.
> 8 is the only cube in the list.
> Can't think of an argument for 13 or 19 just now.
13 is a happy number, but then so is 19. :-)
1² + 3² = 1 + 9 = 10
1² + 0² = 1 + 0 = 1
https://en.wikipedia.org/wiki/Happy_number
OEIS does find anything.
https://oeis.org/search?q=2%2C4%2C6%2C8%2C13%2C19&language=english&go=Search
I did come up with a second answer the other night: four is the only one
with the same number of letters in its spelling as its value.
3 two
→ 4 four
3 six
5 eight
8 thirteen
8 nineteen
But that's not very mathsy, and why give such an awkward sequence when
a simpler one would do?
3 one
3 two
5 three
→ 4 four
4 five
3 six
5 seven
--
Cheers, Ralph.
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