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What are examples of strikingly anomalous phenomena in mathematics, where just one or a rather small number of cases stand out because they don't fit a general pattern?

This is most interesting when the situation considered is very simple and basic, and where the exceptional cases are not merely the lowest-numbered ones.

For example, the outer automorphism of the symmetric group S₆ (which exists for no other S_n), and the existence of non-standard differentiable structures on ℝ⁴ (but no other ℝⁿ).

What are examples of strikingly anomalous phenomena in mathematics, where just one or a rather small number of cases stand out because they don't fit a general pattern?

This is most interesting when the situation considered is very simple and basic and where the exceptional cases are not merely the lowest-numbered ones: for example, the outer automorphism of the symmetric group $S_{6}$ (which exists for no other $S_{n}$), and the existence of non-standard differentiable structures on $\mathbb{R}^{4}$ (but no other $\mathbb{R}^{n}$).

What are examples of strikingly anomalous phenomena in mathematics, where just one or a rather small number of cases stand out because they don't fit a general pattern?

This is most interesting when the situation considered is very simple and basic, and where the exceptional cases are not merely the lowest-numbered ones.

For example, the outer automorphism of the symmetric group S₆ (which exist for no other S_n), and the existence of non-standard differentiable structures on R⁴ (but for no other Rⁿ).

What are examples of strikingly anomalous phenomena in mathematics, where just one or a rather small number of cases stand out because they don't fit a general pattern?

This is most interesting when the situation considered is very simple and basic, and where the exceptional cases are not merely the lowest-numbered ones.

For example, the outer automorphism of the symmetric group S₆ (which exists for no other S_n), and the existence of non-standard differentiable structures on ℝ⁴ (but no other ℝⁿ).