Oscillation properties of the self-adjoint, two term, differential equation $${(-1)}^n{\left(p\left(x\right)y^{\left(n\right)}\right)}^{\left(n\right)}+q\left(x\right)y=0(*)$$ are investigated. Using the variational method and the concept of the principal system of solutions it is proved that (*) is conjugate on $R=(-\infty ,\infty )$ if there exist an integer $m\in \{0,1,\cdots ,n-1\}$ and $c_0,\cdots ,c_m\in R$ such that $${\int }_{\infty }^0{x}^{2(n-m-1)}{p}^{-1}\left(x\right)dx=\infty ={\int }_0^{\infty }{x}^{2(n-m-1)}{p}^{-1}\left(x\right)dx$$ and $$\underset{x_1\downarrow -\infty ,x_2\uparrow \infty }{lim\; sup}{\int }_{x_1}^{x_2}q\left(x\right){(c_0+c_{1}x+\cdots +c_m{x}^m)}^{2}dx\le 0,q\left(x\right)\neg \equiv 0.$$ Some extensions of this criterion are suggested.

New existence results are presented for the two point singular “resonant” boundary value problem $\frac{1}{p}{\left(p{y}^{\text{'}}\right)}^{\text{'}}+ry+{\lambda }_{m}qy=f(t,y,p{y}^{\text{'}})$ a.eȯn $[0,1]$ with $y$ satisfying Sturm Liouville or Periodic boundary conditions. Here ${\lambda }_m$ is the ${(m+1)}^{st}$ eigenvalue of $\frac{1}{pq}[{\left(p{u}^{\text{'}}\right)}^{\text{'}}+rpu]+\lambda u=0$ a.eȯn $[0,1]$ with $u$ satisfying Sturm Liouville or Periodic boundary data.

Existence results are established for the resonant problem $y^{\text{'}\text{'}}+{\lambda }_{m}ay=f(t,y)$ a.e. on $[0,1]$ with $y$ satisfying Dirichlet boundary conditions. The problem is singular since $f$ is a Carathéodory function, $a\in L_{\mathrm{l}oc}^1(0,1)$ with $a>0$ a.e. on $[0,1]$ and ${\int }_0^{1}x(1-x)a\left(x\right)\mathrm{d}x<\infty$.