## Problem 037

**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 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

See also the topics:

Don't post any spoilers

Comments, questions and clarifications about PE problems.

### 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?

### Re: Problem 37 clarification

A number is a member of the list if it is left- and right-truncatable by itself.

### 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

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

### Re: Problem 37 clarification

1 is not considered a prime....

### Re: Problem 37 clarification

D'oh!

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

-Steve

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

-Steve

### Problem 37

For this problem it says to

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.Find the sum of theonlyeleven primes that are both truncatable from left to right and right to left.

### Re: Problem #37

Yes.

### Re: Problem #37

I keep ending up with 24 prime numbers greater than 7 that meet the definition in problem 37.

### Re: Problem #37

Post one five digit prime from your list of 24.

### 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.

### Re: Problem #37

Send the 4 digit numbers via PN to me.

### Re: Problem #37

1 is not a prime number.

### 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...

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 gypaètes sont là.

### Re: Problem 37

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

Edit: Oh, I found the last one!

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

### Re: Problem 37

Hello,

I found 25 trucable primes below ...!!

11

13

.

.

(snip)

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

Please can you give me more informations.

thanks.

I found 25 trucable primes below ...!!

11

13

.

.

(snip)

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

Please can you give me more informations.

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.*### 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

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