The function f(x) takes an input string and replaces vowels (a, e, i, o, u) with the character that comes before them in the English alphabet, and doubles the previous character if it is a consonant.

[CODE]:
def approximate_function(input_string):
    result = ''
    for c in input_string:
        if c in 'aeiou':
            result += chr(ord(c) - 1)
        else:
            result += c
            if c.isalpha() and c.lower() not in 'aeiou':
                result += c
    return result