# How many 8-digit palindromes are prime? [closed]

An integer is said to be palindrome if it reads the same forward or backward. For example, the integer 14541 is a 5 digit palindrome and 12345 is not a palindrome. How many 8-digit palindromes are prime?

## closed as off-topic by Did, Matthew Conroy, user91500, steven gregory, NamasteApr 28 '17 at 23:47

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## 2 Answers

Let the digits of our 8-digit palindrome $n$ be $d_1d_2d_3d_4d_4d_3d_2d_1$. Then the palindrome has the form $$n=10000001\cdot d_1 + 1000010\cdot d_2 + 100100\cdot d_3 + 11000\cdot d_4.$$ Such $n$ cannot be prime because $$\gcd(10000001,1000010,100100,11000)=11.$$ So there are no 8-digit prime palindromes. (However, there are prime palindromes with an odd number of digits, e.g. $101$, $10301$, $98689$, $9801089$.)

I think palindrome is divisible by 11 because division by 11requires addition of alternate digits should be equal let the 8 digit palindrome number be (abcddcba) then a+c+d+b=b+d+c+a hence it cannot be a prime so that means the palindrome having even no of digits cannot be prime number of course 11 is an exception

• Of course Alex what you said is correct . – Ch.Siva Ram Kishore Apr 28 '17 at 5:48