Instanton size modulus
= Instanton size modulus
{title2=$\rho>0$}
The scale $\rho>0$ in a <BPST instanton> changes the width of its action density while leaving total action unchanged. This reflects conformal invariance of four-dimensional classical <Yang-Mills action>. As $\rho\to0$, <gauge curvature> concentrates at the center and the smooth family approaches a bubbling boundary, not another smooth zero-size <instanton>.