For example, consider

*y =*

**sqrt**(

*x*). Suppose you had a graph of

**sqrt**(

*x*) and wanted to show which values of

*x*give an integer result for

*y*, i.e. which values of

*x*are perfect squares:

*y =*

**sqrt**(x) with only integer solutions shown

I recently learned how you can find and display integer solutions for certain types of equations in Desmos, and thought I would write a post about how to do this.

When we care only about the integer solutions of an equation, we refer to it as a Diophantine equation. In general, Diophantine equations can take many forms - the technique described here can be used in a limited class of problems where you can express your Diophantine equation as a function

*k*=*f*(*n*), where*f*takes integers*n*as inputs and want to know which outputs*k*are also integers (*k*> 0).**step 1 - define your real valued function**

Let's see how to apply this method using

**sqrt**(*x*). First, plotting the real-valued*f*(*x*) =**sqrt**(*x*) function, and creating a list of points by evaluating it for a range of integers, shows there are many points on the graph where we put in an integer, and get out a non-integer.
What we do next is to use some of the functions built into Desmos to create an integer detector function, that we can apply to the outputs of

*f*(*x*).**step 2 - create the integer detector function**

We'd like to build a function that can tell if a given input is an integer or not, and then use this to find out when we have an integer value for

*f*(*n*).
We would like to create an indicator function for integers that behaves like this:

To do this, we can make use of some built in functions in Desmos - the

**ceiling**and**floor**functions.
The function

**ceil**rounds a decimal value up to the nearest integer, and**floor**rounds it down to the nearest integer. Consequently,**floor**(*x*) <=**ceil**(*x*), with equality only if*x*is an integer. Note also that for*x*> 0,**ceil**(*x*) != 0, and**floor**(*x*)/**ceil**(*x*) <= 1, with equality only if*x*is an integer. The function**floor**(*x*)/**ceil**(*x*) is almost what we want - it is equal to 1 only when*x*is an integer. To get a function that is zero when*x*is not an integer, we take the**floor**again:
By composing

*t*and*f*, we obtain a function*tf*which tells us when the values of*f*are integers. Plotting*t*(*f*(*x*)) for a range of integer inputs shows us which values of*f*give integer results.*points in red show values for (x, tf(x)), ponts*

in blue show values for (x, f(x)).

in blue show values for (x, f(x)).

The final step is to integrate

*tf*into our graph so that we can plot the integer solutions to*y*=*f*(*x*).**step 3 - taking advantage of undefined**

Desmos handles undefined points gracefully - it just will not plot them. Consequently, if we create a function

*g*, which is identical to*f*where*f*takes on integer solutions, but is undefined when it does not, we can get Desmos to plot exactly the points we want and no others.To create

*g*, we can combine

*f*with our indicator function

*t*:

Plotting points using this new function shows only those which correspond to integer solutions for f(x).

*integer solutions to y = sqrt(x), graph here*

**Another example - Pythagorean triples**

We encounter another example of Diophantine equations when trying to find Pythagorean triples. A Pythagorean triple is an integer solution to the equation a^2 + b^2 = c^2.

At first it might look like we can’t apply the method developed above to the problem of finding Pythagorean triples - there seem to be too many variables in play. However, we can explore the problem within certain ranges by taking advantage of other features that Desmos provides.

*p*.

This allows us to explore Pythagorean triples where one of the legs of the triangle is fixed (determined by the value

*p*). Using the method described above, we can find triples involving p as one of the legs.

*some triples found by Desmos,*

*where p=24 (graph here)*

For example, with

*p*= 24, Desmos finds the triples (24, 7, 25), (24, 18, 30), and others shown in the image above.

*Some older Desmos-related posts:*

*- Brain and Propeller Fractals in Desmos*

*- Polygonal Number Diagrams using Desmos*

*- Spirals in Desmos*

*- Spirolaterals in Desmos*

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