A103664 - OEIS

%I #17 May 31 2019 12:30:46

%S 2,3,5,11,17,23,29,47,53,59,71,79,83,89,107,131,139,149,167,173,179,

%T 191,197,223,227,233,239,251,263,269,293,311,317,347,359,367,383,389,

%U 419,431,439,443,449,461,467,479,499,503,509,557,563,569,587,593,599,607

%N Primes p such that the number of divisors of p-1 is less than the number of divisors of p+1.

%C Mathematica coding by _Wouter Meeussen_ and _Robert G. Wilson v_.

%H Alois P. Heinz, <a href="/A103664/b103664.txt">Table of n, a(n) for n = 1..10000</a>

%e a(1)=2 because d(1)=1 < d(3)=2; a(2)=3 because d(2)=2 < d(4)=3.

%p with(numtheory): p:=proc(n) if isprime(n) and tau(n-1)<tau(n+1) then n else fi end: seq(p(n),n=1..1000); # _Emeric Deutsch_, Feb 22 2005

%t Select[Prime[Range[1, 140]], Length[Divisors[ # - 1]] < Length[Divisors[ # + 1]] &]

%t Select[Prime[Range[200]],DivisorSigma[0,#-1]<DivisorSigma[0,#+1]&] (* _Harvey P. Dale_, May 31 2019 *)

%Y Cf. A008328 number of divisors of p-1, A008329 number of divisors of p+1, A103665, A103666, A103667.

%K nonn

%O 1,1

%A _Hugo Pfoertner_, Feb 19 2005