## Problem 155

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.
Forum rules
As your posts will be visible to the general public you
are requested to be thoughtful in not posting anything
that might explicitly give away how to solve a particular problem.

This forum is NOT meant to discuss solution methods for a problem.

In particular don't post any code fragments or results.

Don't start begging others to give partial answers to problems

Don't ask for hints how to solve a problem

Don't start a new topic for a problem if there already exists one

Don't post any spoilers
viv_ban
Posts: 23
Joined: Mon May 26, 2008 2:09 pm

### Problem 155

Can anybody please confirm me if the values D(4) = 15 and D(5) = 35 are correct.

Tommy137
Posts: 238
Joined: Sun Feb 24, 2008 6:02 pm
Location: Cologne, Germany
Contact:

### Re: problem 155

Yes, they are correct.

viv_ban
Posts: 23
Joined: Mon May 26, 2008 2:09 pm

### Re: problem 155

Thanks Tommy. I really cannot understand the problem with my algorithm. Is D(10) = 2491.

Tommy137
Posts: 238
Joined: Sun Feb 24, 2008 6:02 pm
Location: Cologne, Germany
Contact:

### Re: problem 155

Unfortunately, it's wrong

Schwein
Posts: 3
Joined: Mon Aug 15, 2011 2:57 am

### Re: problem 155

I don't get it either...
Is it wrong to think, that the only new capacitor combinations are the ones from the previous, with max number of capacitors, + a series and a paralel with a new 60 microfahrad capacitor?
ex

if we have 2 2 capacitor combinations, then there will be 4 extra in the next, and 8 extra in the next and so on?

I cannot imagine how else a combination would be made, but then my D(5) = 31, and i don't see where the 4 extra comes from

jaap
Posts: 540
Joined: Tue Mar 25, 2008 3:57 pm
Contact:

### Re: problem 155

Schwein wrote:I cannot imagine how else a combination would be made, but then my D(5) = 31, and i don't see where the 4 extra comes from
Hint: 5 = 1+4 = 2+3
Remember also that we only care about the different values - some values can be made in several completely different ways, so that is another reason why it won't be a simple doubling sequence.

Schwein
Posts: 3
Joined: Mon Aug 15, 2011 2:57 am

### Re: problem 155

I had the part covered, that some values might be equal(turned out none of them were, when i just added one in series, and one in paralel, for each combo), the only thing i don't understand, is why that does not give me all the different combinations makeable...

Lord_Farin
Posts: 239
Joined: Wed Jul 01, 2009 9:43 am
Location: Netherlands

### Re: problem 155

Schwein wrote:the only thing i don't understand, is why that does not give me all the different combinations makeable...
How about combining larger, 'composite' capacitors in series or parallel? (This is what hk meant above as well)
Think about $1 = \frac1{1+1}+\frac1{1+1}$.

Schwein
Posts: 3
Joined: Mon Aug 15, 2011 2:57 am

### Re: problem 155

Ahhhhh, thank you were much the both of you, I see now where my logic fails ^^

Erf_Erf
Posts: 5
Joined: Sun Jan 12, 2014 12:12 pm

### Re: problem 155

It seems I can't get it right... I get the correct results for D(4) and D(5). Then D(6) = 76 and D(10) = 2147. Are these values correct?

mpiotte
Posts: 1914
Joined: Tue May 08, 2012 4:40 pm

### Re: problem 155

Erf_Erf wrote:It seems I can't get it right... I get the correct results for D(4) and D(5). Then D(6) = 76 and D(10) = 2147. Are these values correct?
No.

Erf_Erf
Posts: 5
Joined: Sun Jan 12, 2014 12:12 pm

### Re: problem 155

I just found the annoyingly trivial and almost invisible mistake in my code, and finally got the correct answer.

DeKlod
Posts: 16
Joined: Tue Aug 21, 2018 7:55 am

### Re: problem 155

Hi all,
Could I ask for two pieces of input here:
1) D(13) is a 5-digit figure starting with a 4 and ending in an even digit - is that correct?
2) I'm using float values in C and my suspicion is that my solutions turns out wrong because of a precision issue. I've tried (long) double but the result is no more convincing. Should I try to go for the alternative of defining and manipulating fractions (as Wodehouse would have said: I've just had a look at what my mind is doing, and it turns out: it boggles!) or should I rather look for something else - my choice of value for the capacitance for example.
Cheers,
Claude

Jochen_P
Posts: 54
Joined: Mon Oct 05, 2009 9:47 am
Location: Stuttgart, Germany

### Re: problem 155

Claude,

D(13) is indeed a 5-digit figure but it starts with 3 and ends with an odd digit.

hth
Jochen

DeKlod
Posts: 16
Joined: Tue Aug 21, 2018 7:55 am

### Re: problem 155

Right, back to the drawing board then ...
Thanks for getting back though!

DeKlod
Posts: 16
Joined: Tue Aug 21, 2018 7:55 am

### Re: Problem 155

Right, got there in the end, although the program takes about 40 seconds to come up with the answer...
Thanks for the help again!
Claude