The previous prime function gives the largest prime less than . The function can be given explicitly as
where is the th prime and is the prime counting function. For , 4, ... the values are 2, 3, 3, 5, 5, 7, 7, 7, 7, 11, 11, 13, 13, 13, 13, 17, 17, 19, ... (OEIS A007917).
The previous prime function was implemented in versions of the Wolfram Language prior to 6 as PreviousPrime[n] (after loading the package NumberTheory`NumberTheoryFunctions).
Finding the previous prime before gives the largest -digit prime. For , 2, ..., the first few of these are 7, 97, 997, 9973, 99991, 999983, 9999991, 99999989, ... (OEIS A003618). The amounts by which these are less than are 3, 3, 3, 3, 27, 9, 17, 9, 11, 63, ... (OEIS A033873).