2013? It would be highly unusual for the IRS to question a tax return from 2013. A "math error" in IRS speak likely means the IRS has received tax information from a third party that affects the tax return that you submitted. The IRS changed the tax return based on the new information which causes the return that was submitted to now have a "math error"
TurobTax no longer supports tax year 2013 so cannot assist you with the notice from the IRS.
Which notice did you receive in the mail from the IRS? Was it a CP2000?
If so, go to this IRS website for the CP2000 notice - https://www.irs.gov/individuals/understanding-your-cp2000-notice
The notice you received should give you detailed information on what was changed on your tax return. It also should give you information on what you should do if you either Agree or Disagree with the IRS.
Call the IRS using the phone number that is on the notice to get more information concerning this problem.
