# Python Program for Number of solutions to Modular Equations

Given A and B, the task is to find the number of possible values that X can take such that the given modular equation **(A mod X) = B** holds good. Here, X is also called a solution of the modular equation.

**Examples:**

Input :A = 26, B = 2Output :6ExplanationX can be equal to any of {3, 4, 6, 8, 12, 24} as A modulus any of these values equals 2 i. e.,(26 mod 3) = (26 mod 4) = (26 mod 6) = (26 mod 8) =Output:2Input :21 5Output :2ExplanationX can be equal to any of {8, 16} as A modulus any of these values equals 5 i.e.(21 mod 8) = (21 mod 16) = 5

If we carefully analyze the equation A mod X = B its easy to note that if (A = B) then there are infinitely many values greater than A that X can take. In the Case when (A < B), there cannot be any possible value of X for which the modular equation holds. So the only case we are left to investigate is when (A > B).So now we focus on this case in depth.

Now, in this case we can use a well known relation i.e.

Dividend = Divisor * Quotient + Remainder

We are looking for all possible X i.e. Divisors given A i.e Dividend and B i.e., remainder. So,

We can say, A = X * Quotient + B Let Quotient be represented as Y ∴ A = X * Y + B A - B = X * Y ∴ To get integral values of Y, we need to take all X such that X divides (A - B)∴ X is a divisor of (A - B)

So, the problem reduces to finding the divisors of (A – B) and the number of such divisors is the possible values X can take.

But as we know A mod X would result in values from (0 to X – 1) we must take all such X such that X > B.

Thus, we can conclude by saying that the number of divisors of (A – B) greater than B, are the all possible values X can take to satisfy A mod X = B

`# Python Program to find number of possible` `# values of X to satisfy A mod X = B ` `import` `math` ` ` `# Returns the number of divisors of (A - B)` `# greater than B` `def` `calculateDivisors (A, B):` ` ` `N ` `=` `A ` `-` `B` ` ` `noOfDivisors ` `=` `0` ` ` ` ` `a ` `=` `math.sqrt(N)` ` ` `for` `i ` `in` `range` `(` `1` `, ` `int` `(a ` `+` `1` `)):` ` ` `# if N is divisible by i` ` ` `if` `((N ` `%` `i ` `=` `=` `0` `)):` ` ` `# count only the divisors greater than B` ` ` `if` `(i > B):` ` ` `noOfDivisors ` `+` `=` `1` ` ` ` ` `# checking if a divisor isnt counted twice` ` ` `if` `((N ` `/` `i) !` `=` `i ` `and` `(N ` `/` `i) > B):` ` ` `noOfDivisors ` `+` `=` `1` `;` ` ` ` ` `return` `noOfDivisors` ` ` `# Utility function to calculate number of all` `# possible values of X for which the modular` `# equation holds true ` ` ` `def` `numberOfPossibleWaysUtil (A, B):` ` ` `# if A = B there are infinitely many solutions` ` ` `# to equation or we say X can take infinitely` ` ` `# many values > A. We return -1 in this case ` ` ` `if` `(A ` `=` `=` `B):` ` ` `return` `-` `1` ` ` ` ` `# if A < B, there are no possible values of` ` ` `# X satisfying the equation` ` ` `if` `(A < B):` ` ` `return` `0` ` ` ` ` `# the last case is when A > B, here we calculate` ` ` `# the number of divisors of (A - B), which are` ` ` `# greater than B ` ` ` ` ` `noOfDivisors ` `=` `0` ` ` `noOfDivisors ` `=` `calculateDivisors;` ` ` `return` `noOfDivisors` ` ` ` ` `# Wrapper function for numberOfPossibleWaysUtil() ` `def` `numberOfPossibleWays(A, B):` ` ` `noOfSolutions ` `=` `numberOfPossibleWaysUtil(A, B)` ` ` ` ` `#if infinitely many solutions available` ` ` `if` `(noOfSolutions ` `=` `=` `-` `1` `):` ` ` `print` `(` `"For A = "` `, A , ` `" and B = "` `, B` ` ` `, ` `", X can take Infinitely many values"` ` ` `, ` `" greater than "` `, A)` ` ` ` ` `else` `:` ` ` `print` `(` `"For A = "` `, A , ` `" and B = "` `, B` ` ` `, ` `", X can take "` `, noOfSolutions` ` ` `, ` `" values"` `)` `# main()` `A ` `=` `26` `B ` `=` `2` `numberOfPossibleWays(A, B)` ` ` ` ` `A ` `=` `21` `B ` `=` `5` `numberOfPossibleWays(A, B)` ` ` `# Contributed by _omg` |

**Output:**

For A = 26 and B = 2, X can take 6 values For A = 21 and B = 5, X can take 2 values

Time Complexity of the above approach is nothing but the time complexity of finding the number of divisors of (A – B) ie O(√(A – B))

Please refer complete article on Number of solutions to Modular Equations for more details!