Why is the Digamma function always denoted with the letter "psi"?

Question

My question is on the notation of the Digamma function.

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The Factorial function $n!$ (which is met in secondary school), is conceptually seminal to the Digamma function. The Factorial function is defined as: $$0!=1,\qquad n!=\frac{(n+1)!}{n+1}$$

This concept is extended with Gauss's Pi function: $$\operatorname\Pi(z)=\frac{\operatorname\Pi(z+1)}{z+1}$$ and with a simple unit offset (see here), the more familiar Euler Gamma function: $$\operatorname\Gamma(z)=\frac{\operatorname\Gamma(z+1)}z$$

The Digamma function is a further extension: $$\psi(z)=\dfrac{\operatorname\Gamma'(z)}{\Gamma(z)}$$ and can itself be extended with the Polygamma function: $$\psi^m(z)=\frac{\mathrm d^m}{\mathrm dz^m}\psi(z) %=\frac{\mathrm d^{m+1}}{\mathrm d z^{m+1}}\ln \operatorname\Gamma(z) $$

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All sources I have encountered that mention the Digamma (or Polygamma) denote it as above with the greek letter 'psi' $\psi$. This befuddles and confuses me. Archaic greek's uppercase form of digamma resembles the glyph of capital gamma $\Gamma$, but with and additional horizontal cross bar, making it appear as a 'double gamma' or a latin $\mathrm F$.

The uppercase gamma, and lowercase digamma and psi characters are available in LaTeX/MathJax, where $$\Gamma,\digamma,\psi$$ produces: $$\huge\Gamma,\digamma,\psi$$

It makes sense that 'Digamma' would be chosen as a symbol that is both visually and etymologically similar to the Gamma function, because these functions are closely related. It does not make any sense to chooses the symbol $\psi$. But what really gets me, is that the name of the function is a complete mismatch to the symbol of the function.

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My question(s):

If we are going to call it a 'Digamma' function why don't we use a 'Digamma' symbol?
or conversely
If we are going to use a 'psi' symbol why don't we call it a 'psi' function?

Is there some justification for the establishment or continuation of this entrenched convention? Is this merely an historical artefact of idiotic humanity (a typographical error)? Or is there more to the story?

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My best guess is that it many have something to do with typographical limitations, or overloading. But it still seems to ridiculously confusing notation to learn, (let alone to teach and propagate).

Answer 1

I believe it’s because this function was used, and denoted “psi”, much before it got a name.

Indeed, it looks like $(\log\circ\,\Pi)'$ and $(\log\circ\,\Gamma)'$ first occur in Euler (1755, pp. 797-801; 1769, p. 17), resp. Legendre (1810, p. 502), with no special name or notation. Surveys like Brunel (1886, p. 58; 1899, p. 162) or Jensen (1916, p. 140) agree that they are denoted $\varPsi$ after Gauss (1813, p. 34), resp. $\psi$ after — unclear whom. (Clausen (1858, p. 169) or Bertrand (1870, p. 252) might be early examples.) In any case, they are still unnamed.

Instead, as one learns in Pearson (1922, p. viii), Davis (1933, p. 277; 1935, p. 9; 1935, p. 243), or Jordan (1939, p. 58), the name “digamma” and notation $\digamma$ for $\varPsi$ arose for the first time on p. 5 of

Pairman, Eleanor, Tables of the digamma and trigamma functions. (Tracts for computers, edited by Karl Pearson, Nr. 1.), Cambridge: University Press, 19 S (1919). ZBL47.0510.15:

In short, I think Pairman won her bet to name the yet-unnamed function — but failed to displace the existing “psi” notation. (The only adopters I could find are Jeffreys & Jeffreys (1946, §15.04).)

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Note added: On the other hand, Legendre in his Exercices (1811, p. 19) famously divided elliptic integrals into 3 kinds to be called Nome, Epinome, Paranome: $$ Ϝ=\int\frac{d\varphi}{\Delta},\qquad \mathrm E = \int\Delta\,d\varphi,\qquad \Pi= \int\frac{d\varphi}{(1+n\sin{\!}^2\varphi)\Delta} $$ where $\Delta=\sqrt{1-c^2\sin{\!}^2\varphi}$. When these words didn’t catch on, Verhulst (1841, p. v) justified an attempt to rename them digamma, epsilon, kappa (notation $\mathrm{dig}$, $\mathrm{eps}$, $\mathrm{kap}$) with

1º The letter Ϝ, used by Legendre to denote the function $\int\frac{d\varphi}\Delta$, bears the Greek name δίγαμμα.

$$\Huge\style{font-family:Arial}{\text{Ϝ ϝ 𝟊 𝟋}}$$

Answer 2

According to the French wikipedia page Fonction digamma, it was James Stirling (1730) who first introduced and studied the digamma function, denoting it with the Greek letter digamma (upper case) $\digamma$. However, this claim is marked as requiring a citation. After noting its subsequent study by Legendre, Poisson and Gauss around 1810, the article then simply states "Elle est désormais le plus souvent notée par la lettre $\psi$ (psi minuscule)" - "It is now most often denoted by the letter $\psi$".

After the gamma and digamma functions were introduced they were generalised as the full sequence of polygamma functions. In the context of the polygamma functions it is obviously desirable to have a single functional denotation - i.e., $$\psi^{(n)}(z) := \frac{d^{m+1}}{dz^{(m+1)}} \text{ ln } \Gamma (z)$$

where $\psi^{(0)}$ denotes the digamma function, $\psi^{(1)}$ the trigamma function, etc... In other words, in the context of the sequence of polygamma functions, there is not reason for the digamma function to have a special designation.

If is not clear why psi was chosen, but it seems reasonable to assume that this is why the special $\digamma$ Digamma designation introduced by Stirling fell out of usage. It was simply a matter of notational elegance in the general case.