Stefan Kubiak (25 August 1923 – 28 November 1963), also known as Hồ Chí Toán (nom de guerre "Mathematician"), was a Polish soldier who became a decorated captain in the People's Army of Vietnam.

As a child, he enjoyed reading adventure and historical books, as well as watching films, which distracted him from the poverty of his childhood and caused him to develop an interest in exotic lands.

After witnessing methods of interrogation and torture used by the French forces against Vietnamese peasants, which often resulted in murders and rapes of the civilian population, Kubiak believed he was fighting on the wrong side and decided to desert.

[1][3] Kubiak started his duty in the Việt Minh with the audacious task of dispersing propaganda flyers in Nam Định, the city in which he had previously been stationed with the Legion.

While fighting for the Fu Tong post, he almost died after being hit in the face by a grenade; despite bleeding profusely from his injured head, with the last of his strength he managed to throw off the deadly charge, which then exploded in the enemy bunker.

[3] Within the People's Army of Vietnam, he earned the nickname Toán ("Mathematician") for the exceptional skills in tactics and strategy that he demonstrated during the Điện Biên Phủ campaign.

He was considered a talented soldier, helping to break through the most difficult enemy blockade in the Battle of Điện Biên Phủ by disguising himself as a French officer, which ultimately made it possible for the Vietnamese troops to charge and finally capture the bunker.

[5][1][7][2] After the end of the First Indochina War in 1954, Kubiak was transferred to Hà Nội – the capital city of Vietnam – and continued to work in the military, but now as a reporter for the People's Army Newspaper.

He settled in Hanoi and, in 1956, started a family with a Vietnamese woman called Nguyễn Thị Phượng who was also Catholic; they met thanks to Franciszek Zwierzyński, who was stationed in Vietnam as part of Poland's involvement in the International Control Commission.

Due to recurring wounds sustained in combat and severe malaria, Stefan Kubiak fell gravely ill in 1963 and died in Hanoi, on 28 November of that year.