Follow

Let \(a(1) = 1\).

Define \(a(n) = a(n-1) + n^k\), where \(k\) is the lowest positive integer such that \(a(n-1)+n^k\) has more decimal digits than \(a(n-1)\).

Is it always true that \(a(n)\) has \(n\) digits?

3113440232155803840515 + 23 ^ 16 = 9246050647836802489476

3113440232155803840515 + 23 ^ 17 = 144163479792818772766618

1,17,260,1284,16909,296845,1120388,17897604,405318093,1405318093 ...

n=25 would have been another candidate as counterexample.

(Tried to come up with bounds, there's quite a lot of cases where there's only one possible value of k)

@kat you're right, non-negative k would do

Kat MCP(NT4) MCSE(Win2K)@kat@mastodon.social@christianp

One question, why make k non zero ? perhaps there could be a situation when just adding 1, could increase the number of digits.

Thinking naively, if n=100 then the numbers of digits could easily increase by 2

Also... experimentally ...

I think that a(22) = 3113440232155803840515

which has 22 digits

and

a(23) = 144163479792818772766618

which has 24 digits

See image for haskell...