If you put Peter’s answer above into OEIS, it comes back with some mathematical sequence names. I’m camping so bit tricky to work out on a mobile!
Sent from a mobile device! > On 6 Aug 2025, at 11:20, Ralph Corderoy <[email protected]> wrote: > > Hi, > >>>>> ‘What number is the odd one out, and why: 2 4 6 8 13 19.’ > ... >> Eight, because it is the only one that starts with a vowel. Also not >> very mathsy, but it was a pub quiz so may have nothing to do with >> maths. > > I'm hoping that's not the answer! > > It occurred to me that I'd searched OEIS wrong: I looked for the > sequence 2 4 6 8 13 19 whereas I should have done six searches, one for > each sequence of five numbers remaining after dropping each of the six > in turn. > > But that gives many answers. In other words, they're all the odd one > out, but only for esoteric reasons. For example, dropping 8 to give > 2,4,6,13,19 finds https://oeis.org/A087549: > > a(1) = 1, > a(2) = 1, > a(n) = sum of tau(n) previous terms, where tau(n) is the number of > divisors of n. > > 1, 1, 2, 4, 6, 13, 19, 42, 74 > > > My answer on the spot was 19 is the odd one out because summing the > digits of all the others, and the digits of the answer until one digit > remains, gives even numbers for all of them except 19. > > 2 → 2 > 4 → 4 > 6 → 6 > 8 → 8 > 13 → 1 + 3 = 4 > 19 → 1 + 9 = 10 → 1 + 0 = 1 > > > If I was told the wrong sequence, then another possibility comes from > working backwards given 19 - 13 = 6. > > 19 - 6 = 13 > 13 - 5 = 8 > 8 - 4 = 4 > 4 - 3 = 1 > 1 > > But this would need the question to be about 1,4,6,8,13,19, replacing > the 2 with a 1. Then 6 is the odd one out. But the lady was very > certain of the numbers, so I don't think that's it. > > > So my best answer so far is the digital root. > https://en.wikipedia.org/wiki/Digital_root > > -- > Cheers, Ralph. > > -- > Next meeting: Online, Jitsi, Tuesday, 2025-09-02 20:00 > Check to whom you are replying > Meetings, mailing list, IRC, ... https://dorset.lug.org.uk > New thread, don't hijack: mailto:[email protected] -- Next meeting: Online, Jitsi, Tuesday, 2025-09-02 20:00 Check to whom you are replying Meetings, mailing list, IRC, ... https://dorset.lug.org.uk New thread, don't hijack: mailto:[email protected]
