 Solution

The finite angular resolution of COBE DMR suppresses small angular scales, so only low spherical harmonic multipoles can be recovered usefully. The approximate relation $\theta \sim \pi /l$ associates $l\simeq 17$ with scales of order ten degrees; this is an effective sensitivity and beam-window limit, not a mathematical hard truncation.

Interpret the printed superscript uncertainty as $(1.1\pm 0.1)\times 10^{-5}$. Its square estimates the band-limited zero-separation variance, rather than the full $C(0)$:Use the coefficient supplied in the paper for this numerical inference. With ${\sigma }_{2:17}=1.1\times 10^{-5}$,so c givesThe stated amplitude uncertainty gives approximately $18\%$ relative uncertainty in $A$, or $A\simeq (1.38\pm 0.25)\times 10^6\mathrm{Mp}{\mathrm{c}}^4$, keeping $H_0$ fixed. The ideal matter-era Sachs-Wolfe plateau with precisely the c spectrum convention has coefficient $2\pi /9=0.698$, rather than $0.732$; the supplied numerical coefficient is about $5\%$ larger and has been treated as the paper's prescribed approximation. A real beam-convolved analysis retains its window function and additional cosmological transfer effects.