Error messages on viewing blog

I get these 3 messages when opening my blog:
Warning: fopen(/home/rcentor/public_html/wp-content/links-update-cache.xml): failed to open stream: Permission denied in /home/rcentor/public_html/wp-includes/links-update-xml.php on line 79
Warning: fwrite(): supplied argument is not a valid stream resource in /home/rcentor/public_html/wp-includes/links-update-xml.php on line 80
Warning: fclose(): supplied argument is not a valid stream resource in /home/rcentor/public_html/wp-includes/links-update-xml.php on line 81
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public_html/wp-content/links-update-cache.xml is an empty file with permissions 644
Does anyone know who to prevent these error messages?
thanks
To view the errors:
http://medrants.com