A023521 - OEIS

%I #18 Jun 04 2020 01:05:02

%S 5,9,19,21,73,18,32,111,42,451,196,381,883,108,93,526,266,232,72,2593,

%T 36,162,1236,98,112,752,55,26,3081,55,4161,1002,9523,135,1616,444,863,

%U 1368,117,415,266,3464,2642,1908,1172,3504,1312,2538,135

%N Sum of distinct prime divisors of prime(n)*prime(n-1) - 1.

%C If n-1 is in A103746, then a(n) = (prime(n)*prime(n-1)+3)/2. - _Robert Israel_, Jun 03 2020

%H Robert Israel, <a href="/A023521/b023521.txt">Table of n, a(n) for n = 2..10000</a>

%F a(n) = A008472(A023515(n)). - _Michel Marcus_, Sep 30 2013

%p p:= 2: R:= NULL:

%p for n from 2 to 50 do

%p   q:= p; p:= nextprime(p);

%p   R:= R, convert(numtheory:-factorset(p*q-1),`+`)

%p od:

%p R; # _Robert Israel_, Jun 03 2020

%o (PARI) sopf(n) = my(fac=factor(n)); sum(i=1, matsize(fac)[1], fac[i, 1]) ;

%o a(n) = sopf(prime(n)*prime(n-1) - 1); \\ _Michel Marcus_, Sep 30 2013

%Y Cf. A103746.

%K nonn,look

%O 2,1

%A _Clark Kimberling_

%E Offset set to 2 and a(1) removed by _Michel Marcus_, Sep 30 2013