Ei

Ei is a term with several distinct meanings across languages and disciplines. It can be a common noun in German, a mathematical function, or a given name in Japan, among other usages. The different meanings are not etymologically related and depend on context.

In German, Ei means "egg." It is used in everyday language with phrases like "ein Ei" for one egg and the plural form "Eier" for multiple eggs. As a basic vocabulary item, it also features in cooking, food labeling, and language instruction.

In mathematics, Ei denotes the exponential integral function. For real x, it is defined by Ei(x) = - PV ∫_{-x}^{∞} (e^{-t}/t) dt, where PV stands for the Cauchy principal value. An equivalent representation is Ei(x) = γ + ln|x| + ∑_{n=1}^∞ x^n/(n·n!) for x ≠ 0, with γ the Euler-Mascheroni constant. The derivative satisfies d/dx Ei(x) = e^x/x for x ≠ 0. The function is not elementary and arises in problems involving integrals of exponential functions divided by a variable, appearing in physics, engineering, and applied mathematics.

In Japanese, Ei is used as a given name and can be found in romanized spellings of several individuals and fictional characters. As a syllable, "ei" also occurs broadly in Japanese names and words.

Because these meanings span different domains, the correct interpretation of "Ei" relies on surrounding context.