include jquery plugin file not working

Your uses of wp_register_script and add_action are incorrect. Try the following code:

function you_fancy_js(){
    wp_register_script( 'custom-script', plugins_url( '/js/jquery.js', __FILE__ ), array( 'jquery', 'jquery-ui-core', 'jquery-validate' ) );  
    wp_enqueue_script( 'custom-script' );
}
add_action('wp_enqueue_scripts','you_fancy_js');

This assumes that your custom jQuery functions are in jquery.js and jquery, jquery-ui-core, and jquery-validate are already enqueued. You do not need admin_head. The admin equivalent of wp_enqueue_scripts is admin_enqueue_scripts.

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