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### Mini-Challenge #2: Square numbers with all digits 0 to 9 How many square numbers use all the digits 0 to 9 once and once only?If you think you've solved it, or if you're stuck, email pi3challenge@sheffield.ac.uk or tweet to @pi3challengeA square number is one that can be written as the square of another number. For example, 9 is a square number, because 9 = 32 = 3 x 3.An example of a number with all the digits once only is 9586714230. But this is not a square number. An example of a number with all the digits once only which is also a square number is 1532487609 = 391472.Hint #1 : Whereas Challenge #1 needed some subtle mathematical reasoning, this challenge just needs your programming skills. Hint #2 : The `str`, `set`, and `len` functions are very useful for this problem. Take a look at this page on lists, strings and sets. Find out how a number can be turned into a string which can then be turned into a list or set. Extra challenges: How many prime numbers can be written as N+1, where N is a square number with all digits.How many prime numbers can be written as N-1, where N is a square number with all digits.  Is this just coincidence, or can you find an explanation?How many of the roots n of these square numbers, n^2 = N, are themselves prime numbers? Coincidence, or can you find an explanation?How many permutations of the digits 9876543210 are there? How many of these are valid numbers? What proportion of such numbers are square numbers?Can you find an example of a number whose square contains each digit exactly twice?