A078150 Smallest k such that d(phi(k)) - phi(d(k)) = n, where d(k) = A000005(k) and phi(k) = A000010(k).
3, 5, 7, 17, 13, 35, 31, 37, 113, 77, 61, 221, 185, 143, 211, 209, 181, 287, 241, 577, 1729, 403, 421, 1297, 1057, 1001, 2113, 779, 1009, 899, 1321, 1917, 5629, 1333, 1801, 2233, 7125, 1763, 2161, 2993, 4433, 4851, 3737, 3311, 51319, 2623, 2521
Offset: 1
Programs
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Mathematica
f[x_] := DivisorSigma[0, EulerPhi[x]]-EulerPhi[DivisorSigma[0, x]] t=Table[0, {100}]; Do[s=f[n]; If[s<101&&t[[s]]==0, t[[s]]=n], {n, 1, 100000}]; t -
PARI
a(n) = my(k=1); while (numdiv(eulerphi(k)) - eulerphi(numdiv(k)) != n, k++); k; \\ Michel Marcus, Jun 19 2025