gistfile1.php

<?php
if(is_archive()) {
  function rel_next_prev(){
      global $paged;
  
      if ( get_previous_posts_link() ) { ?>
          <link rel="prev" href="<?php echo get_pagenum_link( $paged - 1 ); ?>" /><?php
      }
  
      if ( get_next_posts_link() ) { ?>
          <link rel="next" href="<?php echo get_pagenum_link( $paged +1 ); ?>" /><?php
      }
  
  }
  add_action( 'wp_head', 'rel_next_prev' );
}
?>