[tex]U(2^{42} + 3^{42}) = \\
=U(2^{42}) + U(3^{42}) = \\
=U(2^{4\times 10+2}) + U(3^{4\times 10+2}) = \\
=U\Big[\Big(2^{4}\Big)^{10}\times 2^2}\Big] +U\Big[\Big(3^{4}\Big)^{10}\times 3^2}\Big] = \\
=U\Big[16^{10}\times 2^2}\Big] +U\Big[81^{10}\times 3^2}\Big] = \\
=U\Big[6\times 4}\Big] +U\Big[1\times 9}\Big] = \\
U( 24 + 9) = U(33) = \boxed{3}
[/tex]
