def get_word_freq_in_sentences(word, sentences):
    """
        :param word: the word which frequency we calculate
        :param sentences: a list of the sentences, representing the document / search space
        :return: the number of occurrences of the given word in the search space. Letter case is ignored
    """
    freq = 0
    for sentence in sentences:
        for w in sentence:
            if str(word).lower() == str(w).lower():
                freq += 1

    return freq