Only One

There is only one palindromic prime with an even number of digits.

Find it.


The answer is 11.

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6 solutions

Danish Ahmed
Jan 19, 2015

Since each palindrome with an even number of digits is divisible by 11 11 - the only palindromic prime with an even number of digits is 11 11 .

I guess what we need to prove is that 11 is the only palindrome with even number digits.

Roman Frago - 6 years, 4 months ago

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The way this question is worded means that we don't. The question states that there is only one of these numbers. We found one, therefore it is the only one.

Louis W - 6 years, 4 months ago

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The problem assumes that, but how do you prove the assumption is correct?

Roman Frago - 6 years, 3 months ago

what about 1331?

Júlio Vinicius Rodrigues Miguel - 6 years, 4 months ago

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1331 = 1 1 3 1331=11^3

Daniel Liu - 6 years, 4 months ago

its not prime

Aakash Khandelwal - 6 years, 4 months ago

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fair enought. haushaus

Júlio Vinicius Rodrigues Miguel - 6 years, 4 months ago

where is the even number?

Sanjoy Roy - 6 years, 4 months ago

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the number is not even . the number should have even number of digits

Danish Ahmed - 6 years, 4 months ago

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There are 2 digits.

Roman Frago - 6 years, 4 months ago

What is a palindromic prime ?

Ahmed H - 6 years, 4 months ago

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palindromic means it is same number if you read it from left to right and from right to left. For example, number 1235321 is palindromic. And prime means it is divisible only by 1 and itself.

Aleksandar Banjac - 6 years, 4 months ago
Jack Han
Jan 29, 2015

Every palindromic number with an even number of digits is divisible by 11. 11 is only divisible by 1 and itself therefore 11 is both palindromic, prime and has an even number of digits.

Paulo Carlos
Jan 29, 2015

The smallest prime palindromic numbers are: 2, 3, 5, 7, 11, 101, 131, 151, 181, 191, 313, 353, 373, 383, 727, 757, 787, 797, 919, 929, 10301, 10501, 10601, 11311, 11411, 12421, 12721, 12821, 13331, 13831, 13931, 14341, 14741, 15451, 15551, 16061, 16361, 16561, 16661, 17471, 17971, 18181, 18481, 19391, 19891, 19991, ...

The only that satisfy the requirements is: 11; Then, the answer is 11 \boxed {11}

Young Champ
Feb 2, 2015

A palindrome is a number that reads the same from left to right and from right to left

For example, 1331, 202 ,12345654321,33,121

Further, a prime is a number which has only two factors ie. 1 and itself.

A palindromic prime (sometimes called a palprime) is a prime number that is also a palindromic number. Palindromicity depends on the base of the numbering system and its writing conventions, while primality is independent of such concerns. The first few decimal palindromic primes are: 2, 3, 5, 7, 11, 101, 131, 151, 181, 191, 313, 353, 373, 383, 727, 757, 787, 797, 919, 929, …

Except for 11, all palindromic primes have an odd number of digits, because the divisibility test for 11 tells us that every palindromic number with an even number of digits is a multiple of 11.

Thus, 11 is the answer to the question

Mohit Tubid
Jan 30, 2015

Every palindromic number with an even number of digits is divisible by 11 . And we know 11 is only divisible by 1 and itself therefore 11 is both palindromic, prime and has an even number of digits...... simple is it...

Ayush Tyagi
Jan 30, 2015

11 is a prime number with even number of digits.

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