1.00000000000000000015
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As written, it would just be 1.00000000000000000015. However, if you meant 0.00000000000000000015 then there are eighteen zeros between the decimal point and the first non-zero digit, so it would be written as 1·5×10^-19 or 1·5E-19. The original version, which you wrote, could more succinctly be written as 1 + 1·5E-19.
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that's the best you're going to be able to do. i don't believe that 1 + 1.5*10^-19 is acceptable, but perhaps