do  -- find some factorial primes - primes that are f - 1 or f + 1 for some factorial f

    local fmt = require( "fmt" )      -- RC Pluto formatting library
    local int = require( "int" )      -- RC Pluto integer library, inc. prime utilities

    local f, fpCount, n = 1, 0, 0
    while fpCount < 10 do
        n += 1
        f *= n
        local fpOp = "-"
        for fp = f - 1, f + 1, 2 do
            if int.isprime( fp ) then
                fpCount += 1
                fmt.write( "%2d:%4d! %s 1 = %d\n", fpCount, n, fpOp, fp )
            end
            fpOp = "+"
        end
    end

end