Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #9 Jun 13 2020 03:23:12
%S 6,9,12,14,18,20,33,44,62,70,92,108,116,138,164,175,212,254,280,308,
%T 320,332,348,356,452,490,524,558,572,692,716,764,833,932,956,1004,
%U 1105,1124,1172,1188,1436,1496,1562,1593,1676,1724,1772,1964,2002,2036,2088,2132
%N Numbers k such that psi(k) = psi(k + 2) where psi(k) is the Dedekind psi function (A001615).
%H Amiram Eldar, <a href="/A330703/b330703.txt">Table of n, a(n) for n = 1..10000</a>
%H Jozsef Sandor, <a href="https://www.emis.de/journals/JIPAM/article546.html">On the composition of some arithmetic functions, II</a>, Journal of Inequalities in Pure and Applied Mathematics, Vol. 6, No. 3 (2005), Article 73.
%e 6 is in the sequence since psi(6) = psi(8) = 12.
%t psi[1] = 1; psi[n_] := n * Times @@ (1 + 1/Transpose[FactorInteger[n]][[1]]); Select[Range[10^3], psi[#] == psi[# + 2] &]
%Y Cf. A001494, A001615, A007373, A062832, A291043.
%K nonn
%O 1,1
%A _Amiram Eldar_, Dec 26 2019