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%I #19 Oct 18 2019 11:25:47
%S 6,42,1806,2786,47058,73178,85082,2143066830,2214502422,3138798830,
%T 4404051298,4428107218,4428595298
%N Composite numbers n such that (n'+1)' = n', where n' is the arithmetic derivative of n.
%C All primes are a solution to this equation. For composite numbers, a(8) > 10^7.
%C No more terms below 4.8*10^10. - _Amiram Eldar_, Oct 18 2019
%t dn[0]=0; dn[1]=0; dn[n_] := Module[{f=Transpose[FactorInteger[n]]}, If[PrimeQ[n], 1, Plus@@(n*f[[2]]/f[[1]])]]; Select[Range[2,100000], !PrimeQ[#] && dn[#] == dn[dn[#]+1]&]
%Y Cf. A003415, A203618 (solutions of the differential equation (n'-1)' = n-1).
%K nonn,more
%O 1,1
%A _José María Grau Ribas_, Jan 24 2012
%E a(8)-a(13) from _Amiram Eldar_, Oct 18 2019
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