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%I #9 Jan 16 2023 11:34:33
%S 4,8,9,18,25,27,28,57,62,72,85,123,128,176,184,189,192,218,220,234,
%T 243,246,252,256,258,259,261,278,282,306,309,316,322,332,338,339,356,
%U 375,380,388,399,403,490,495,505,512,518,544,590,597,622,632,655,662
%N sigma(n) - phi(n) is a palindrome greater than 2.
%C When n is prime sigma(n)-phi(n) is 2, so that case is trivial.
%H Harvey P. Dale, <a href="/A116030/b116030.txt">Table of n, a(n) for n = 1..1000</a>
%e sigma(399) - phi(399) = 424.
%t pg2Q[n_]:=With[{c=DivisorSigma[1,n]-EulerPhi[n]},PalindromeQ[c]&&c>2]; Select[ Range[700],pg2Q] (* _Harvey P. Dale_, Jan 16 2023 *)
%o (Magma) [n: n in [1..1000] | Intseq(d) eq Reverse(Intseq(d)) and d gt 2 where d is DivisorSigma(1,n)-EulerPhi(n)]; // _Bruno Berselli_, Sep 09 2015
%Y Cf. A115897, A116031.
%K nonn,base
%O 1,1
%A _Giovanni Resta_, Feb 13 2006