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%I #20 Aug 13 2019 07:04:39
%S 10,20,40,50,80,210,420,630,711,831,840,2910,29910,40320,80640,98361,
%T 673140,865580,8656341,466760130,694602930,821412711,23465346510,
%U 40396039620,63130473930,234000006510,464205665820,2340653406510,2346599346510
%N Numbers n such that phi(n)=4*reversal(n).
%C If p=10^m-3 is prime then 30*p is in the sequence because phi(30*p)=phi(30)*phi(p)=8*(10^m-4)=4*(2*10^m-8)=4*reversal (3*10^m-9)=4*reversal(3*p)=4*reversal(30*p). Next term is greater than 55*10^7.
%C Let f(m,n)=(78*10^(m+3)+210)*(10^(n*(m+4))-1)/(10^(m+4)-1)+7, if p=f(m,n) is prime then 30*p is a term of the sequence. - _Jahangeer Kholdi_, Nov 13 2013
%C Also if p=(1/101)*(680*10000^n+27) is prime then 60*p is in the sequence. - _Jahangeer Kholdi_, Nov 13 2013
%C a(30) > 10^13. - _Giovanni Resta_, Aug 12 2019
%e 20 is in the sequence because phi(20)=4*2=4*reversal(20).
%t Do[If[EulerPhi[n]==4*FromDigits[Reverse[IntegerDigits[n]]], Print[n]], {n, 550000000}]
%Y Cf. A000010, A004086, A069215, A114930, A136538.
%K nonn,base,more
%O 1,1
%A _Farideh Firoozbakht_, Jan 29 2006
%E a(21)-a(27) from _Giovanni Resta_, Oct 28 2012
%E a(28)-a(29) from _Giovanni Resta_, Aug 12 2019
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