Here's the configuration file I use to help me enter some of the fun mathematical unicode characters with vim. The commands were borrowed from TeX...

imap jj <ESC>
imap \div ÷
imap \times ×
imap \sum ∑
imap \int ∫
imap \oint ∮
imap \angle ∠
imap \forall ∀
imap \exists ∃
imap \partial ∂
imap \prod ∏
imap \infty ∞
imap \pm ±
imap \sqrt √
imap \circ ∘
imap \Re ℛ
imap \Im ℑ

imap \' ′
imap \aleph ℵ
imap \nat ℕ
imap \real ℝ
imap \rat ℚ
imap \Int ℤ
imap \null ∅
imap \del ∇
imap \cdot ⋅
imap \therefore ∴
imap \simeq ≃
imap \approx ≈
imap \cong ≅
imap \ne ≠
imap \equiv ≡
imap \leq ≤
imap \geq ≥
imap \in ∈
imap \oplus ⊕
imap \ominus ⊖
imap \otimes ⊗
imap \oslash ⊘
imap \odot ⊙
imap \cap ∩
imap \cup ∪
imap \bigcup ⋃
imap \uplus ⊎
imap \star ⋆
imap \neg ¬
imap \iff ⇔
imap \leftarrow ←
imap \uparrow ↑
imap \rightarrow →
imap \downarrow ↓
imap \Leftarrow ⇐
imap \Uparrow ⇑
imap \Rightarrow ⇒
imap \Downarrow ⇓
imap \alpha α
imap \beta β
imap \gamma γ
imap \delta δ
imap \epsilon ε
imap \zeta ζ
imap \eta η
imap \theta θ
imap \iota ι
imap \kappa κ
imap \lambda λ
imap \mu μ
imap \nu ν
imap \xi ξ
imap \pi π
imap \rho ρ
imap \sigma σ
imap \tau τ
imap \phi φ
imap \chi χ
imap \psi ψ
imap \omega ω
imap \Gamma Γ
imap \Delta Δ
imap \Theta Θ
imap \Lambda Λ
imap \Omega Ω
imap \lchv «
imap \rchv »
imap \^0 ⁰
imap \^1 ¹
imap \^2 ²
imap \^3 ³
imap \^4 ⁴
imap \^5 ⁵
imap \^6 ⁶
imap \^7 ⁷
imap \^8 ⁸
imap \^9 ⁹
imap \^i ⁱ
imap \^n ⁿ
imap \^+ ⁺
imap \^- ⁻
imap \^= ⁼
imap \^( ⁽
imap \^) ⁾
imap \_0 ₀
imap \_1 ₁
imap \_2 ₂
imap \_3 ₃
imap \_4 ₄
imap \_5 ₅
imap \_6 ₆
imap \_7 ₇
imap \_8 ₈
imap \_9 ₉
imap \_+ ₊
imap \_- ₋
imap \_= ₌
imap \_( ₍
imap \_) ₎
imap \1/2 ½
imap \1/4 ¼
imap \3/4 ¾
imap \1/3 ⅓
imap \2/3 ⅔
imap \1/8 ⅛
imap \3/8 ⅜
imap \5/8 ⅝
imap \7/8 ⅞
imap \! ¡
imap \-> u20d1
imap \^ u0302