Online Algebra Calculator
Many of users do not use powerful Maxima computer algebra system for systematic work, but for ad hoc algebraic calculations – equations, functions, matrixes, etc. only. To do this job, it is not useful to download the whole application from a web site.
Function: prev_prime (<n>) Returns the greatest prime smaller than <n>.

(%i1) prev_prime(27);
(%o1) 23(%o1) true (%i2)