In mathematics, the Prouhet–Thue–Morse constant, named for Eugène Prouhet [fr], Axel Thue, and Marston Morse, is the number—denoted by τ—whose binary expansion 0.01101001100101101001011001101001... is given by the Prouhet–Thue–Morse sequence. That is,

${\displaystyle \tau =\sum _{n=0}^{\infty }{\frac {t_{n}}{2^{n+1}}}=0.412454033640\ldots }$

where t_n is the n^th element of the Prouhet–Thue–Morse sequence.

The Prouhet–Thue–Morse constant can also be expressed, without using t_n , as an infinite product,^[1]

${\displaystyle \tau ={\frac {1}{4}}\left[2-\prod _{n=0}^{\infty }\left(1-{\frac {1}{2^{2^{n}}}}\right)\right]}$

This formula is obtained by substituting x = 1/2 into generating series for t_n

${\displaystyle F(x)=\sum _{n=0}^{\infty }(-1)^{t_{n}}x^{n}=\prod _{n=0}^{\infty }(1-x^{2^{n}})}$

The continued fraction expansion of the constant is [0; 2, 2, 2, 1, 4, 3, 5, 2, 1, 4, 2, 1, 5, 44, 1, 4, 1, 2, 4, 1, …] (sequence A014572 in the OEIS)

Yann Bugeaud and Martine Queffélec showed that infinitely many partial quotients of this continued fraction are 4 or 5, and infinitely many partial quotients are greater than or equal to 50.^[2]

The Prouhet–Thue–Morse constant was shown to be transcendental by Kurt Mahler in 1929.^[3]

He also showed that the number

${\displaystyle \sum _{i=0}^{\infty }t_{n}\,\alpha ^{n}}$

is also transcendental for any algebraic number α, where 0 < |α| < 1.

Yann Bugaeud proved that the Prouhet–Thue–Morse constant has an irrationality measure of 2.^[4]

The Prouhet–Thue–Morse constant appears in probability. If a language L over {0, 1} is chosen at random, by flipping a fair coin to decide whether each word w is in L, the probability that it contains at least one word for each possible length is ^[5]

${\displaystyle p=\prod _{n=0}^{\infty }\left(1-{\frac {1}{2^{2^{n}}}}\right)=\sum _{n=0}^{\infty }{\frac {(-1)^{t_{n}}}{2^{n+1}}}=2-4\tau =0.35018386544\ldots }$

• Euler-Mascheroni constant
• Fibonacci word
• Golay–Rudin–Shapiro sequence
• Komornik–Loreti constant