factor: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/factor - from package: coreutils

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/factor - from package: coreutils
/usr/lib/plan9/bin/factor - from package: 9base
/usr/lib/phylip/bin/factor - from package: phylip

You may install the required package with command apt-get install {package}

Error in other languages:
factor: bevel nie gevind nie
factor: comandă negăsită
factor: comando não encontrado
factor: commande introuvable
factor: command not found
factor: käsku ei ole
factor: không tìm thấy lệnh
factor: komanda nerasta
factor: Kommando nicht gefunden.
factor: kommandot finns inte
factor: komut yok
factor: nie znaleziono polecenia
factor: níor aimsíodh an t-ordú
factor: no se encontró la orden
factor: no s'ha trobat l'ordre
factor: opdracht niet gevonden
factor: parancs nem található
factor: perintah tidak ditemukan
factor: príkaz nenájdený
factor: příkaz nenalezen
factor: команда не найдена
factor: командата не е открита
factor: 命令找不到
factor: コマンドが見つかりません