There are online graphing calculators. However, as of writing, there is no emulated TI83 that runs within your browser. On the other hand, if you have a Ti83, it is possible to copy its ROM to run an emulated TI83 on your desktop via one of many emulators.
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All versions of Windows operating system since at least win98 have included a calculator which had a scientific mode. For example in Windows 8 open the calculator then click on the file menu. You will see Standard, Scientific, Programmer, and Statistics. Just select the mode you would like to use.
Caution: Most offers for free calculator downloads are not really free and unencumbered.
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