Donaldson (1983) showed there exists an exotic smooth differential structure on 
. Donaldson's result has been extended
to there being precisely a continuum of nondiffeomorphic
differential structures on 
.
Exotic R^4
See also
Exotic Sphere, Smooth StructureExplore with Wolfram|Alpha
References
Donaldson, S. K. "Self-Dual Connections and the Topology of Smooth 4-Manifold." Bull. Amer. Math. Soc. 8, 81-83, 1983.Monastyrsky, M. Modern Mathematics in the Light of the Fields Medals. Wellesley, MA: A K Peters, 1997.Cite this as:
Weisstein, Eric W. "Exotic R^4." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/ExoticR4.html