Luitzen Egbertus Jan Brouwer (1881–1966) was an early worker in algebraic topology and the founder of intuitionism.
fan theorem and bar theorem.
On intuitionism:
Scans and transcription of his nine PhD-era notebooks, largely in Dutch:
which were studied and analysed in
From p. 6 of Errett Bishop‘s Foundations of Constructive Analysis (McGraw-Hill, 1967):

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