A cobordism from a closed oriented -manifold to is a compact oriented -manifold whose oriented boundary is .
Start with and attach an -dimensional one-handle to one -ball in each component of its outgoing boundary. The new outgoing boundary is , so this is a cobordism from to their connected sum.

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Cobordism is a concept from the field of topology, particularly in algebraic topology, that studies the relationships between manifolds. In simple terms, cobordism provides a way to classify manifolds based on their boundaries and their relationships to each other.