Talk:216 (number)

Untitled

the sum of the divisors of 216 is equal to 600' Skyjetter (talk) 22:44, 12 June 2011 (UTC)

Explanation of removal of content

I have removed the content of a previous edit: http://en.wikipedia.org/w/index.php?title=216_%28number%29&action=historysubmit&diff=242604872&oldid=242097195

Looking at that edit, there are nine sequences of 24 numbers that follow a clear progression. The first two terms in each sequence are the same digit repeated, from 1 to 9 for each of the sequences. Subsequent terms are derived by adding the preceding two terms and, if the result is greater than 9, summing the digits (always returning a value from 1 to 9, since none of the sequences start "0 0" - a degenerate case).

For any such sequence with starting values a and b (each a digit from 1 to 9), the values at each step will be as follows:

step	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26
a	1	0	1	1	2	3	5	8	4	3	7	1	8	9	8	8	7	6	4	1	5	6	2	8	1	9
b	0	1	1	2	3	5	8	4	3	7	1	8	9	8	8	7	6	4	1	5	6	2	8	1	9	1

Trivially, the digits of any integer greater than 0 that has been multiplied by 9 will reduce to 9. And adding 9 to any integer will not change the value down to which its digits reduce. Therefore you can see that the sequence will always repeat from the 25th step since (a+9b) will reduce to a and (9a+b) will reduce to b.

For the sequences that start with the same two digits, the 24th value will always be 9, after which the sequences repeat. So the sequences in that edit show the 24 digits that make up these sequences before they repeat. As it happens, 24×9=216. But this is concidental since using a different base other than base-10 will lead to a different number of sequences of different lengths. Similarly allowing for starting values that are not equal will increase the number of sequences.

It is more important that such a sequence repeats with a period of 24 (in base-10) rather than that there happen to be 24×9=216 individual digits in this subset of the complete set of possible sequences. Certainly this is not a useful fact to add to the page about the number 216.

And further this is still not an important enough fact for it to be considered as an addition to the page about the number 24 either. Considering the periodicity of such sequences in other bases, it can be seen that although 24 is a common period (in the sample I have calculated here) it is certainly not the only possible value:

Base:	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20
Periodicity:	3	8	6	20	24	16	12	24	60	10	24	28	48	40	24	36	24	18

Please note that the above explanation is here not for inclusion in Wikipedia (since it currently consitutes original research) but rather as an explanation as to why I have removed the content of the edit that I noted above.

-Stelio (talk) 11:44, 10 November 2009 (UTC)

I removed it again. — Arthur Rubin (talk) 07:48, 23 February 2017 (UTC)

Semiprimes

It is claimed that 216 is the only number n such that $n-3,n-2,n-1,n+1,n+2,n+3$ are all semiprimes. This seems unlikely. If n = 1260 m + 216, we would need only have the following numbers prime.

• 420 m + 71
• 630 m + 107
• 252 m + 43
• 180 m + 31
• 630 m + 109
• 420 m + 73

Clearly, none of these are divisible by 2 or 3, and at most one is divisible by each of 5 or 7. Hence, for any prime p, we can choose m (mod p) such that none of the six numbers above are divisible by p. If, for example, the Schinzel's hypothesis H holds, there are infinitely many m such that all 6 are prime, and hence n = 1260 m + 216 satisfies the required condition.

This, of course, is WP:OR, and not, itself, suitable for Wikipedia. It's just an explanation why a proof is unlikely. — Arthur Rubin (talk) 15:38, 11 July 2017 (UTC)

However, OEIS reports 143100 is a solution, so I edited the article to "smallest" from "only". — Arthur Rubin (talk) 20:29, 11 July 2017 (UTC)