## Problem 037

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kvom
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Location: Georgia, USA

### Problem 037

I think it must be assumed that the "subprimes" of the example number 3797 are not to be counted as part of the 11. In other words, 797 is not one of the 11 primes in the solution. Correct?
hk
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Location: Haren, Netherlands

### Re: Problem 37 clarification

A number is a member of the list if it is left- and right-truncatable by itself.
schveiguy
Posts: 3
Joined: Tue Jan 22, 2008 6:35 pm

### Re: Problem 37 clarification

I'm also very confused on problem 37. My solution finds 25 primes that match the description. Here are the first 11 of my solution (I figure it's not going to help anyone since the answer is wrong). Could someone please tell me why any of these numbers doesn't match? Just say the number is wrong and I'll see if I can figure out why.

11 - 1 is prime
13 - 1 and 3 are prime
17 - 1 and 7 are prime
31 - 1 and 3 are prime
37 - 3 and 7 are prime
53 - 3 and 5 are prime
71 - 1 and 7 are prime
73 - 7 and 3 are prime
113 - 11, 13, 1, and 3 are prime
131 - 13, 31, and 1 are prime
137 - 13, 37, 1, and 7 are prime

-Steve
hk
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### Re: Problem 37 clarification

1 is not considered a prime....
schveiguy
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Joined: Tue Jan 22, 2008 6:35 pm

### Re: Problem 37 clarification

D'oh!

Oh yeah, that eliminates most of my answers I got it now.

-Steve
joffray
Posts: 4
Joined: Mon Jul 28, 2008 1:19 am

### Problem 37

For this problem it says to
Find the sum of the only eleven primes that are both truncatable from left to right and right to left.
Are two digit primes included in this eleven? Because I'm ending up with a lot more than eleven primes that meet this definition. Any clarification is appreciated.
Georg
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### Re: Problem #37

Yes.
joffray
Posts: 4
Joined: Mon Jul 28, 2008 1:19 am

### Re: Problem #37

I keep ending up with 24 prime numbers greater than 7 that meet the definition in problem 37.
Georg
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### Re: Problem #37

Post one five digit prime from your list of 24.
joffray
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Joined: Mon Jul 28, 2008 1:19 am

### Re: Problem #37

I'm not even up in to the five digit primes yet. Within my list I have 9 primes that are 2 digits, 10 primes that are 3 digits, and 5 that are 4 digits. I can't see what the problem is because I've gone through the numbers and they seem correct.
Georg
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Contact:

### Re: Problem #37

Send the 4 digit numbers via PN to me.
Georg
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### Re: Problem #37

1 is not a prime number.
joffray
Posts: 4
Joined: Mon Jul 28, 2008 1:19 am

### Re: Problem #37

d'oh
uws8505
Posts: 58
Joined: Tue Sep 30, 2008 3:13 pm
Location: South Korea

### Problem 37

The problem says that there are 11 truncatable primes, but I get only 9 under 1000000, or 10^6.

37
73
...

Are there any truncatable primes over 10^8, 10^9, or 10^10?

If 10^10 is the case, I should run my laptop overnight to get the answer with my algorithm...
Math and Programming are complements
daniel.is.fischer
Posts: 2400
Joined: Sun Sep 02, 2007 11:15 pm
Location: Bremen, Germany

### Re: Problem 37

You haven't found all below one million. I say nothing about where you can find the largest.
Il faut respecter la montagne -- c'est pourquoi les gypa&egrave;tes sont l&agrave;.
uws8505
Posts: 58
Joined: Tue Sep 30, 2008 3:13 pm
Location: South Korea

### Re: Problem 37

Well, thanks, and I found another one. But I cannot get any further...

Edit: Oh, I found the last one!
Last edited by rayfil on Sat Oct 11, 2008 3:46 am, edited 2 times in total.
Reason: Edited to not give away any limit (rayfil)
Math and Programming are complements
near
Posts: 2
Joined: Fri Oct 17, 2008 11:01 pm

### Re: Problem 37

Hello,

I found 25 trucable primes below ...!!

11
13
.
.
(snip)

why 11 and 13 ... are not truncable primes??

thanks.
Last edited by rayfil on Sat Oct 18, 2008 4:54 am, edited 1 time in total.
Reason: Removed some of the irrelevant info.
jaap
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### Re: Problem 37

1 is not a prime.
near
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Joined: Fri Oct 17, 2008 11:01 pm

### Re: Problem 37

thank you Jaap
phiroze
Posts: 1
Joined: Sat Nov 08, 2008 6:33 pm

### problem 37 : on truncatable primes [1373 left-truncatable?]

Hello,
I seem to have struggled with this problem for way too long.
is 1373 a left-truncatable and right-truncatable prime or not?

left_truncate(1373) => 1373, 137, 13, 1 ; all of which are prime
right_truncate(1373) => 1373, 373, 73, 3 ; all of which are prime

but http://www.research.att.com/~njas/sequences/A024785 does not list 1373 as left-truncate prime!

quite confused,...

-- phiroze