// post previous and next navigation stays within same category - look for TRUE argument added below
 
if ( ! function_exists( 'twentythirteen_post_nav' ) ) :
/**
 * Display navigation to next/previous post when applicable.
*
* @since Twenty Thirteen 1.0
*/
function twentythirteen_post_nav() {
	global $post;
 
	// Don't print empty markup if there's nowhere to navigate.
	$previous = ( is_attachment() ) ? get_post( $post->post_parent ) : get_adjacent_post( false, '', true );
	$next     = get_adjacent_post( false, '', false );
 
	if ( ! $next && ! $previous )
		return;
	?>
	<nav class="navigation post-navigation" role="navigation">
		<h1 class="screen-reader-text"><?php _e( 'Post navigation', 'twentythirteen' ); ?></h1>
		<div class="nav-links">
 
			<?php previous_post_link( '%link', _x( '<span class="meta-nav">&larr;</span> %title', 'Previous post link', 'twentythirteen' ), TRUE ); ?>
			<?php next_post_link( '%link', _x( '%title <span class="meta-nav">&rarr;</span>', 'Next post link', 'twentythirteen' ), TRUE ); ?>
 
		</div><!-- .nav-links -->
	</nav><!-- .navigation -->
	<?php
}
endif;