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Composite numbers k such that k*sigma(k) == 2 (mod phi(k)).
1

%I #20 Oct 14 2023 21:10:17

%S 4,6,22

%N Composite numbers k such that k*sigma(k) == 2 (mod phi(k)).

%C Composite terms of A051948.

%D J. Roberts, Lure of the Integers, Math. Assoc. America, 1992, p. 147.

%D M. V. Subbarao, On two congruences for primality. Pacific J. Math. 52 (1974), 261-268.

%t Select[Range[4,100], ! PrimeQ[#] && Mod[#*DivisorSigma[1, # ] - 2 , EulerPhi[#]] == 0 &] (* _Jean-François Alcover_, Sep 16 2013 *)

%Y Cf. A051948.

%K nonn,fini,full,bref

%O 1,1

%A _N. J. A. Sloane_

%E Definition corrected by _Tanya Khovanova_, Nov 05 2007