A random pattern or an insane function

There is a sequence which has the first 3 terms as

1 , 4201 , 94095 , 1, 4201, 94095, \ldots .

The 99th term has all but one digits. Call the missing digit x x . Find the reverse of the digits of x x th term.


The answer is 60466176.

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3 solutions

Vinay Sipani
May 18, 2014

Let f(z) be a function defined on z such that it forms the palindrome of z. E g : f ( 1234 ) = 4321 Eg:f(1234)=4321 We have, 1st term = f ( 1 10 ) = 1 \text{1st term}=f(1^{10})=1 2nd term = f ( 2 10 ) = 4201 \text{2nd term}=f(2^{10})=4201 3rd term = f ( 3 10 ) = 94095 \text{3rd term}=f(3^{10})=94095 . . . . . . 99th term = f ( 9 9 10 ) \text{99th term}=f(99^{10})

From Binomial theorem,on expansion of 99th term,it is found that it doesn't has 6 => x = 6 6th term = f ( 6 10 ) \text{6th term}= f(6^{10}) Hence,Palindrome of 6th term is f ( f ( 6 10 ) ) = 6 10 = 60466176 f(f(6^{10})) = 6^{10}= 60466176

That is not the definition of the word "palindrome". The correct term should be "reverse the digits".

Calvin Lin Staff - 7 years ago

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I thought palindrome could be defined as what I wrote it as. Sorry, I'll change the wording.

Sharky Kesa - 7 years ago

yes...you are right.👍

Vinay Sipani - 7 years ago

Can someone explain how you got the fact that 99^10 doesn't contain the digit 6? I had no idea how to work it out, eventually, I used a calculator, which felt like cheating.

I tried using representing 99 as (100 - 1) to try and make it easier to multiply, but that didn't help too much

Kartik Prabhu - 7 years ago

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You could use the algorithm that is often used by computer software, which is essentially a kind of divide-and-conquer, if you will.

  • calculate 99 squared
  • use this result to calculate 99 cubed
  • use this and the previous result to calculate 99 to the fifth
  • square the previous result to obtain 99 to the tenth

Bill Bell - 5 years, 11 months ago
Vineeth Chelur
May 18, 2014

Brilliant problem. The word palindrome gave it away. Or else I would never have figured out the sequence. Reversing the digits of the numbers in the given sequence, we get 1 , 1024 , 59049 1,1024,59049 Observe that they are all 10th powers of natural numbers 1 10 , 2 10 , 3 10 1^{10},2^{10},3^{10} Since the 99th term is to be found, use wolframalpha to find the value 9 9 10 = 904382075008804490001 99^{10} = 904382075008804490001 Observe that the digit 6 is missing. So the 6th term will be the reverse of 6 10 6^{10} But the question asks us to find the palindrome of the 6th term, which will simply be 6 10 = 60466176 6^{10} = \boxed{60466176}

Bill Bell
Jul 17, 2015

Confession: I used oeis.org to identify the sequence. (I'd still be peering at this problem tomorrow if I hadn't.)

Here f calculates terms in the sequence in the form of strings rather than integers. I verify that the digit 6 is not in the string for f ( 99 ) f(99) . Then I calculate f ( 6 ) f(6) and its value with digits reversed.

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