1.

 Punctured 8 neighbourhood of a.

Answer»
  • N * (a, δ) = {x | (a – δ) < x < (a + δ), x ≠ a, x∈ R}
  • N (a, δ) = |x – a|< δ (Modulus form)
  • N (a, δ) = (a – δ, a + δ) (Interval form)


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