He also helped devise the Weierstrass–Erdmann condition, which gives sufficient conditions for an extremal to have a corner along a given extrema, and allows one to find a minimizing curve for a given integral.
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This became a topic of his Master's thesis titled "On the expansion of transcendental function in partial fractions. After publishing his thesis Bukreev went abroad and took lectures of Karl Weierstrass, Lazarus Fuchs, and Leopold Kronecker in Berlin. Bukreev undertook research on Fuchsian functions under Fuchs' guidance, which he completed in 1888 and which became the basis of his doctoral thesis "On the Fuchsian functions of rank zero" defended in 1889.
In the second half of the nineteenth century, the calculus was reformulated by Augustin-Louis Cauchy, Bernard Bolzano, Karl Weierstrass, Cantor, Dedekind, and others using the (ε, δ)-definition of limit and set theory.
Disillusioned with the state of mathematical research in North America at the time, he left for Europe in 1891, locating primarily in Berlin, Göttingen and Paris, where he associated with some of the greatest mathematical minds of the time, including Karl Weierstrass, Felix Klein, Ferdinand Georg Frobenius and Max Planck.