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Reverse Square to Prime (Posted on 2010-12-14) Difficulty: 3 of 5

No Solution Yet Submitted by brianjn    
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Solution Ruling out possibilities Comment 1 of 1
Call n the number that is to be squared.

n must be 32 to 99
n cannot be divisible by 3 or 11 or the reversal wont be prime
also n^2 cannot begin with 2,4,5,6,or 8 or the reversal wont be prime
[we are down to 19 possibilities]

when a number is added to its reversal the result is divisible by 11, to be a semiprime it cannot be divisible by 2 or 5.
So the first and last digits of n^2  cannot be both odd, both even, or add to 5.
This leaves 7 possibilities for n
{34, 40, 56, 58, 62, 86, 98}

Checking which n^2 are prime when reversed rules out five of these.  So the 2 left are
{40, 62}

You must be allowing leading zeros, since these are the two solutions:

40^2 = 1600.  0061 = 61 which is prime, and 1600+61 = 1661 = 11*151

62^2 = 3844.  4483 is prime and 3844+4483 = 8327 = 11*757

  Posted by Jer on 2010-12-14 17:17:43
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