def extract_arxiv_url(system_control_numbers):
    """Extract any arxiv URLs from the system_control_number field

    """
    if isinstance(system_control_numbers, dict):
        system_control_numbers = [system_control_numbers,]
    arxiv_urls = [
        number['value'].replace('oai:arXiv.org:', 'https://arxiv.org/abs/')
        for number in system_control_numbers if number.get('institute', '') == 'arXiv' and 'value' in number
    ]
    if not arxiv_urls:
        return ''
    return arxiv_urls[0]