HP originally created this app and it is based on the original calculator ROM. I still have my old 15C and did not hesitate to purchase this app when it came out at a far higher price than most other calculator apps on the Play store. After almost 40 years, this calculator still holds its own for quick, easy, and accurate operation, and the interface works as well with touch as it did with the tactile buttons (which have never worn out). This is an old-school Reverse Polish Notation (RPN) calculator. If you love to have full control over your computations and in exactly what order things are done, with an eye on precision or the effects of rounding, and the thought of not having to input parenthesis brings joy to your soul, this may become the best calculator experience you've ever had. There are newer, graphic calculators out, but for sheer number crunching and fast input, nothing beats the old XYZT RPN stack that the 15C was among the last of HP's calculators to use. (Newer models had much larger stacks which made calculations with constants much more awkward if you are familiar with how the 4-element stack works). Because you control the operator precedence explicitly, It helps you develop skills for recognizing the innermost parts of equations quickly, even very complicated ones. Those skills have helped me do some incredible performance optimizations in work otherwise unrelated to this calculator. Matrix operations are simple and extremely fast, and there's a crib sheet for advanced but slightly cryptic operations on the back of the physical calculator that they've duplicated in the app with a simple press of the menu button. It also includes a PDF of the original owner's manual, which includes an RPN primer as well as some high quality tutorials on algebra, matrix algebra, and other areas that can help reinforce material currently being learned in school or just refresh fading memories. Plus it's searchable and in full color. RPN does have a learning curve, so if you're someone who doesn't like "different," this may not be your thing. But if this write-up has left you curious, give it a shot. I switched to RPN as a senior in High School decades ago and never looked back.