Problem 037

kvom · Post by **kvom** » Mon Oct 08, 2007 6:33 pm

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 · Post by hk » Mon Oct 08, 2007 8:25 pm

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

schveiguy · Post by **schveiguy** » Thu Jan 24, 2008 6:30 pm

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 · Post by hk » Thu Jan 24, 2008 7:23 pm

1 is not considered a prime....

schveiguy · Post by **schveiguy** » Thu Jan 24, 2008 9:22 pm

D'oh!

Oh yeah, that eliminates most of my answers

I got it now.

-Steve

joffray · Post by **joffray** » Mon Jul 28, 2008 1:24 am

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 · Post by **Georg** » Mon Jul 28, 2008 1:40 am

joffray · Post by **joffray** » Mon Jul 28, 2008 1:45 am

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

Georg · Post by **Georg** » Mon Jul 28, 2008 1:47 am

Post one five digit prime from your list of 24.

joffray · Post by **joffray** » Mon Jul 28, 2008 1:56 am

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 · Post by **Georg** » Mon Jul 28, 2008 1:59 am

Send the 4 digit numbers via PN to me.

Georg · Post by **Georg** » Mon Jul 28, 2008 2:04 am

1 is not a prime number.

joffray · Post by **joffray** » Mon Jul 28, 2008 2:13 am

uws8505 · Post by **uws8505** » Wed Oct 08, 2008 2:37 am

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...

daniel.is.fischer · Post by **daniel.is.fischer** » Wed Oct 08, 2008 2:56 am

You haven't found all below one million. I say nothing about where you can find the largest.

uws8505 · Post by **uws8505** » Wed Oct 08, 2008 11:14 am

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

Edit: Oh, I found the last one!

near · Post by **near** » Fri Oct 17, 2008 11:08 pm

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.

jaap · Post by **jaap** » Fri Oct 17, 2008 11:27 pm

1 is not a prime.

near · Post by **near** » Sat Oct 18, 2008 1:13 am

thank you Jaap

phiroze · Post by **phiroze** » Sat Nov 08, 2008 6:43 pm

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

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Problem 037

Problem 037

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Problem 37

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Problem 37

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