## Problem 521

A place to air possible concerns or difficulties in understanding ProjectEuler problems. This forum is not meant to publish solutions. This forum is NOT meant to discuss solution methods or giving hints how a problem can be solved.
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DeatH_StaR
Posts: 13
Joined: Sat Apr 19, 2014 5:09 pm

### Problem 521

Hi all,
Can problem 521 be solved by the "one minute rule"?
Just *counting* to 1012 took me on a fast computer more then 15 minutes (not finding primes, just counting).
So will I need to expect a slow solution, or am I missing something that will enable me to solve it fastly?
TripleM
Posts: 382
Joined: Fri Sep 12, 2008 3:31 am

### Re: Problem 521

Every problem satisfies the 1 minute rule. If you can't count to 10^12 in one minute, then clearly you shouldn't be trying to
DeatH_StaR
Posts: 13
Joined: Sat Apr 19, 2014 5:09 pm

### Re: Problem 521

Thanks, I got worried...
v6ph1
Posts: 128
Joined: Mon Aug 25, 2014 7:14 pm

### Re: Problem 521

Even if you can not count from 1 to 10^12 within the 1 minute rule, it is possible to get the sum of all these numbers within less than 1 minute.
The same is true for some number theoretic functions.
So you may calc that sum in a different way.

-- v6ph1

PS: Simply sieving needs around 7 hours on a modern i7 CPU.
Marcus_Andrews
Posts: 1523
Joined: Wed Nov 09, 2011 5:23 pm

### Re: Problem 521

DeatH_StaR wrote:Hi all,
Can problem 521 be solved by the "one minute rule"?
Just *counting* to 1012 took me on a fast computer more then 15 minutes (not finding primes, just counting).
So will I need to expect a slow solution, or am I missing something that will enable me to solve it fastly?
We don't release problems unless we can confirm they are solvable in under a minute:

I've written my program but should it take days to get to the answer?
Absolutely not! Each problem has been designed according to a "one-minute rule", which means that although it may take several hours to design a successful algorithm with more difficult problems, an efficient implementation will allow a solution to be obtained on a modestly powered computer in less than one minute.
vamsikal3
Posts: 89
Joined: Sat Oct 01, 2016 9:25 am

### Re: Problem 521

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Last edited by vamsikal3 on Fri Nov 27, 2020 3:00 am, edited 2 times in total.
my friend key --> 990813_OZPwQtCjkD6KlvxirOoTSZxccMFsuw1L
philiplu
Posts: 40
Joined: Wed Aug 02, 2017 8:51 pm
Location: Redmond, WA, USA

### Re: Problem 521

vamsikal3 wrote: Tue Nov 13, 2018 3:57 am Can someone verify the answer for n = 201820182018 is <snipped by moderator>? Thanks!
Those are the correct final 9 digits.
vamsikal3
Posts: 89
Joined: Sat Oct 01, 2016 9:25 am

### Re: Problem 521

<deleted post>
Last edited by vamsikal3 on Fri Nov 27, 2020 3:00 am, edited 2 times in total.
my friend key --> 990813_OZPwQtCjkD6KlvxirOoTSZxccMFsuw1L
kenbrooker
Posts: 187
Joined: Mon Feb 19, 2018 3:05 am
Location: Northern California, USA

### Re: Problem 521

Ahhh... Silly me... I thought --
"This forum is not meant to publish solutions."
"In particular don't post any ... results."
"Good Judgment comes from Experience;