Michael Stifel or Styfel (1487 – April 19, 1567) was a German monk, Protestant reformer and mathematician.

Tensions in the abbey grew after he published the poem Von der Christförmigen, rechtgegründeten leer Doctoris Martini Luthers (1522, i.e. On the Christian, righteous doctrine of Doctor Martin Luther) and came into conflict with Thomas Murner.

Stifel then left for Frankfurt, and soon went to Mansfeld, where he began his mathematical studies.

[1] Due to the tense situation in the Archduchy of Austria in the wake of the execution of Leonhard Kaiser in Schärding, Stifel returned to Wittenberg in 1527.

At this time Stifel started writing a book collecting letter transcripts of Martin Luther, completed in 1534.

He also performed the "Wortrechnung" (i.e. word-calculation), studying the statistical properties of letters and words in the bible (a common method at that time).

He studied "Die Coss" (the first algebra book written in German) by Christoph Rudolff and Euclid's Elements in the Latin edition by Campanus of Novara.

Jacob Milich supported his scientific development and encouraged him to write a comprehensive work on arithmetic and algebra.

So he wrote, that regarding this issue marvelous books could be written, but he himself will refrain and keep his eyes shut.

The letters A, M, A/S, I, A/S each are representing a single operation step when solving a quadratic equation.

[12][13] Another topic dealt with in the Arithmetica integra are negative numbers (which Stifel calls numeri absurdi).

Further issues were a method of calculating roots of higher order by using binomial coefficients[14] and sequences.