下列函数中，是对数函数的是___.
①\( y={\mathrm{log}}_{\frac{1}{2}}x\left(b>0，\mathrm{且}b\ne 1\right)\)；② \( y={\mathrm{log}}_{a}x\left(b>0，\mathrm{且}b\ne 1\right)\)；③\( y={\mathrm{log}}_{a}{x}^{2}(a>0\)，且 \( a\ne 1)\)
④ \( y={\mathrm{log}}_{\left(\sqrt{2}-1\right)}x+1\)；⑤\( y={\mathrm{log}}_{2}\left(\frac{x}{2}\right)\)；⑥ \( y={\mathrm{log}}_{x}2(x>0\)，且 \( x\ne 1)\)
