cp's OEIS Frontend

A192281 Prime numbers whose anti-divisors are all prime numbers.

%I A192281 #7 Jul 22 2025 12:11:54
%S A192281 5,7,11,17,19,29,43,47,61,71,79,89,101,107,109,151,191,197,223,251,
%T A192281 271,317,349,359,421,439,461,521,569,601,631,659,673,691,701,719,811,
%U A192281 821,881,911,919,947,971,991,1009,1051,1091,1109,1153,1181,1217,1231
%N A192281 Prime numbers whose anti-divisors are all prime numbers.
%e A192281 Anti-divisors of 109 are 2, 3, 7, 31, 73 that are all prime numbers
%p A192281 with(numtheory);
%p A192281 P:=proc(i)
%p A192281 local a,b,c,k,j,n,ok;
%p A192281 for n from 3 by 1 to i do
%p A192281   j:=ithprime(n); a:={};
%p A192281    for k from 2 to j-1 do
%p A192281      if abs((j mod k)- k/2) < 1 then a:=a union {k}; fi;
%p A192281    od;
%p A192281    b:=nops(a); c:=op(a);
%p A192281    if b>1 then
%p A192281      ok:=1;
%p A192281      for k from 1 to b do
%p A192281        if (not isprime(c[k])) then ok:=0; break; fi;
%p A192281      od;
%p A192281      if ok=1 then print(j); fi;
%p A192281    else
%p A192281      if isprime(op(a)) then print(j); fi;
%p A192281    fi;
%p A192281 od;
%p A192281 end:
%Y A192281 Cf. A066272
%K A192281 nonn,easy
%O A192281 1,1
%A A192281 _Paolo P. Lava_, Jul 15 2011