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 A055164 (n - phi(n)) | sigma(n) for composite n not congruent to 2 (mod 4). 2
 99, 168, 187, 493, 637, 780, 943, 1273, 1537, 1836, 2183, 2225, 2976, 3103, 3589, 4183, 5353, 5928, 6201, 6468, 6667, 8881, 9553, 9727, 13393, 13888, 14453, 15397, 17587, 19897, 24253, 24883, 30883, 33667, 36259, 36853, 37523, 43657, 45901 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Let p be an odd prime. If d > p+1 is a divisor of p^2 - p - 2 such that d+1-p is prime, then p*(d+1-p) is in the sequence. - Robert Israel, May 03 2019 LINKS Robert Israel, Table of n, a(n) for n = 1..500 MAPLE filter:= proc(n) uses numtheory; if isprime(n) then return false fi; type(sigma(n)/(n-phi(n)), integer) end proc: select(filter, [seq(seq(4*i+j, j=[0, 1, 3]), i=1..20000)]); # Robert Israel, May 03 2019 MATHEMATICA Do[ If[ !PrimeQ[ n ], If[ Mod[ n, 4 ]! = 0, If[ Mod[ DivisorSigma[ 1, n ], n-EulerPhi[ n ] ] == 0, Print[ n ] ] ], {n, 1, 5000} ] PROG (Sage) [n for n in (1..50000) if not mod(n, 4)==2 and not is_prime(n) and mod(sigma(n), n - euler_phi(n))==0] # G. C. Greubel, May 03 2019 CROSSREFS Cf. A000010, A000203, A068422. Sequence in context: A325881 A185498 A126230 * A075815 A075814 A320707 Adjacent sequences: A055161 A055162 A055163 * A055165 A055166 A055167 KEYWORD easy,nonn AUTHOR Robert G. Wilson v, Jun 30 2000 STATUS approved

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Last modified September 11 08:57 EDT 2024. Contains 375814 sequences. (Running on oeis4.)