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Four shared values do not force equality
Example
Assume Countable Choice. The distinct nonconstant entire functions and share exactly the four distinct sphere values ignoring multiplicity: both omit and , and their preimage sets of and of agree, No other sphere value is shared. Since , the five distinct shared values required by the five-value uniqueness theorem Nevanlinna five-value uniqueness theorem cannot be reduced to four.
Facts & Assumptions
Given: The functions and ; Countable Choice is assumed as in the statement, and the computation below is choice-free.
Five-value theorem: if two nonconstant meromorphic functions on share five distinct sphere values ignoring multiplicity, that is, the preimage sets of each value agree, then they are identically equal (Nevanlinna five-value uniqueness theorem).
Fibres of the exponential: , and exactly when ; moreover (, and exactly when ).
The complex exponential is entire and (The complex exponential is entire and its complex derivative is itself, , , and ).
The complex exponential maps onto (The complex exponential maps onto ).
Sharing ignoring multiplicity concerns the sets of points only: counts each preimage once and multiplicities are discarded (Truncated value and ramification counts), exactly the convention in the statement of [F1].
Verification
(Omitted values) By [F3] both and are entire and never vanish; hence the preimage set of is empty for both, and the preimage set of , that is, the pole set, is empty for both as well. Thus and are shared values in the sense of [F5] with empty preimage sets.
(The value ) By [F2], if and only if ; and if and only if , which is the same set. Hence the two -point sets agree and equal .
(The value ) By [F2] and , if and only if , that is, ; and if and only if , that is, as well, because . Hence the two -point sets agree.
(Distinctness) If then , hence , so by [F2] the number would lie in ; but is real and nonzero while every element of is purely imaginary. Thus .
(No other shared value) Let be a shared value in the sense of [F5]. Since we may fix with by [F4]; the preimage sets of under and are and by [F2], and agreement forces , hence ; then by [F2], so , contrary to the choice of . Hence the shared sphere values of and are exactly , four in number.
(Sharpness) Steps 1.1-1.5 exhibit two distinct nonconstant meromorphic functions sharing exactly four distinct sphere values ignoring multiplicity, while [F1] guarantees equality as soon as five distinct values are shared. Therefore the number five in the five-value theorem is optimal.
Depends on
- Nevanlinna five-value uniqueness theorem
- Truncated value and ramification counts
- The Axiom of Countable Choice ($\mathrm{AC}_\omega$)
- $\ker(\exp)=2\pi i\mathbb Z$, and $\exp z=\exp w$ exactly when $z-w\in2\pi i\mathbb Z$
- The complex exponential is entire and its complex derivative is itself
- $\exp(x+iy)=e^x(\cos y+i\sin y)$, $|\exp(x+iy)|=e^x$, and $e^{i\pi}+1=0$
- The complex exponential maps $\mathbb C$ onto $\mathbb C\setminus\{0\}$
Used by
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Dependency tree · two levels
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Sources
- I. Laine, Complex Analysis III lecture notes (standard reference, not scraped)