= Solution
Diagonalizing the linearized blocking map gives perturbations $v_i'=\lambda_iv_i=b^{y_i}v_i$. A <relevant operator> has $y_i>0$, equivalently $|\lambda_i|>1$ for a discrete scaling multiplier: its perturbation grows under repeated coarse-graining and must be tuned to reach the critical fixed point. An <irrelevant operator> has $y_i<0$ and decays, usually contributing corrections to scaling. A <marginal operator> has $y_i=0$, so linearization alone is insufficient; nonlinear flow can make it marginally relevant or irrelevant, or exactly marginal.
For an ordinary spin-inversion-symmetric critical point the temperature and magnetic field are relevant. Fixing $h=0$ by symmetry leaves the temperature to tune. At a <tricritical point> the quartic coupling is another relevant tuning direction. A <dangerously irrelevant coupling> is an exception to simply discarding small variables: setting it to zero can destabilize the ordered potential or make its scaling function singular, even though it flows toward zero.
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