matho-sum: 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 Debian possible default full paths are:

/usr/bin/matho-sum - from package: mathomatic-primes

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

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