This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A124273 #21 Oct 21 2018 06:24:54
%S A124273 3,7,13,17,19,31,47,59,61,71,101,103,107,109,137,149,151,157,167,197,
%T A124273 211,223,227,229,269,317,337,349,353,379,383,389,401,421,439,449,457,
%U A124273 463,479,521,523,541,547,563,569,571,587,599,613,617,631,643,647,677
%N A124273 Primes p that divide A124271(p) = Sum_{i=1..p} (prime(i)^p - 1) / (prime(i) - 1).
%C A124273 a(n) almost coincides with A123856(n). Up to 1000 there are only 3 terms of A123856(n) that are different from the terms of a(n), see A124275.
%H A124273 M. F. Hasler, Nov 10 2006, <a href="/A124273/b124273.txt">Table of n, a(n) for n = 1..199</a>
%p A124273 A124271_mod:=proc(n) option remember; local s,i,p; s:=0: for i to n do p:=ithprime(i) mod n: if p<>1 then s:=s+(p&^n - 1)/(p - 1) mod p fi od:s end; A124273 := proc(n::posint) option remember; local p; if n>1 then p:=nextprime( procname(n-1)) else p:=2 fi: while A124271_mod(p)<>0 do p:=nextprime( p ) od: p end # _M. F. Hasler_, Nov 10 2006
%t A124273 Select[Prime@ Range@ 125, Divisible[Sum[(Prime[i]^# - 1)/(Prime[i] - 1), {i, 1, #}], #] &] (* _Michael De Vlieger_, Jul 17 2016 *)
%o A124273 (PARI) isA124273(p) = isprime(p)&&!sum(i=1, p, sum(j=0, p-1, Mod(prime(i), p)^j)) \\ _Jianing Song_, Oct 20 2018
%Y A124273 Cf. A123856, A124271, A124274 (nonprimes n that divide A124271(n)), A124275 (terms of A123856 that are not in this sequence).
%K A124273 nonn
%O A124273 1,1
%A A124273 _Alexander Adamchuk_, Oct 23 2006