运行 sudo apt-get update 时出现以下错误。有人告诉我这是因为某种名为 ppa 的东西。我如何检查添加了 ppa 的内容或它来自哪里，以便在不需要它时可以将其删除？
我最近正在下载 CUDA 8.0https://developer.nvidia.com/cuda-80-ga2-download-archive
(test1) user@user-P15SM:~/Downloads$ sudo apt-get update
Get:1 file:/var/cuda-repo-8-0-local-ga2 InRelease
Ign:1 file:/var/cuda-repo-8-0-local-ga2 InRelease
Get:2 file:/var/cuda-repo-8-0-local-ga2 Release [574 B]
Get:2 file:/var/cuda-repo-8-0-local-ga2 Release [574 B]
Hit:4 https://download.docker.com/linux/ubuntu xenial InRelease
Hit:5 http://security.ubuntu.com/ubuntu xenial-security InRelease
Hit:6 http://sg.archive.ubuntu.com/ubuntu xenial InRelease
Ign:7 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial InRelease
Hit:8 http://sg.archive.ubuntu.com/ubuntu xenial-updates InRelease
Hit:9 http://sg.archive.ubuntu.com/ubuntu xenial-backports InRelease
Ign:10 http://downloads.skewed.de/apt/xenial xenial InRelease
Ign:11 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial Release
Get:12 http://downloads.skewed.de/apt/xenial xenial Release [2,419 B]
Get:13 http://downloads.skewed.de/apt/xenial xenial Release.gpg [833 B]
Ign:13 http://downloads.skewed.de/apt/xenial xenial Release.gpg
Ign:14 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main amd64 Packages
Hit:15 http://downloads.skewed.de/apt/xenial xenial/universe Sources
Hit:16 http://downloads.skewed.de/apt/xenial xenial/universe amd64 Packages
Hit:17 http://downloads.skewed.de/apt/xenial xenial/universe i386 Packages
Ign:18 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main i386 Packages
Ign:19 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main all Packages
Ign:20 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main Translation-en_SG
Ign:21 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main Translation-en
Ign:22 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main amd64 DEP-11 Metadata
Ign:23 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main DEP-11 64x64 Icons
Ign:14 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main amd64 Packages
Ign:18 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main i386 Packages
Ign:19 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main all Packages
Ign:20 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main Translation-en_SG
Ign:21 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main Translation-en
Ign:22 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main amd64 DEP-11 Metadata
Ign:23 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main DEP-11 64x64 Icons
Ign:14 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main amd64 Packages
Ign:18 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main i386 Packages
Ign:19 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main all Packages
Ign:20 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main Translation-en_SG
Ign:21 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main Translation-en
Ign:22 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main amd64 DEP-11 Metadata
Ign:23 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main DEP-11 64x64 Icons
Ign:14 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main amd64 Packages
Ign:18 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main i386 Packages
Ign:19 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main all Packages
Ign:20 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main Translation-en_SG
Ign:21 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main Translation-en
Ign:22 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main amd64 DEP-11 Metadata
Ign:23 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main DEP-11 64x64 Icons
Ign:14 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main amd64 Packages
Ign:18 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main i386 Packages
Ign:19 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main all Packages
Ign:20 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main Translation-en_SG
Ign:21 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main Translation-en
Ign:22 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main amd64 DEP-11 Metadata
Ign:23 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main DEP-11 64x64 Icons
Err:14 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main amd64 Packages
404 Not Found
Ign:18 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main i386 Packages
Ign:19 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main all Packages
Ign:20 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main Translation-en_SG
Ign:21 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main Translation-en
Ign:22 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main amd64 DEP-11 Metadata
Ign:23 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial/main DEP-11 64x64 Icons
Fetched 3,252 B in 30s (108 B/s)
Reading package lists... Done
W: The repository 'http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu xenial Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: GPG error: http://downloads.skewed.de/apt/xenial xenial Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7A80C8ED4FCCBE09
W: The repository 'http://downloads.skewed.de/apt/xenial xenial Release' is not signed.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: Failed to fetch http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu/dists/xenial/main/binary-amd64/Packages 404 Not Found