ubuntu12.04 更新完源 出现 GPG error

W: GPG error: http://mirror.bjtu.edu.cn precise/ Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 51716619E084DAB9
W: Failed to fetch http://cran.rstudio.com/bin/linux/ubuntu/maverick/Packages  404  Not Found [IP: 54.230.73.11 80]

E: Some index files failed to download. They have been ignored, or old ones used instead.
hadoop@ubuntu:~$ gpg --keyserver pgpkeys.mit.edu --recv-key 51716619E084DAB9（这个16进制的数字串就是上边错误提示中NO_PUBKEY 51716619E084DAB9）
gpg: requesting key E084DAB9 from hkp server pgpkeys.mit.edu
gpg: key E084DAB9: public key "Michael Rutter <[email protected]>" imported
gpg: Total number processed: 1
gpg:               imported: 1  (RSA: 1)
hadoop@ubuntu:~$ gpg -a --export 51716619E084DAB9| sudo apt-key add -
OK
hadoop@ubuntu:~$ sudo apt-get update（重新update）