A023521 Sum of distinct prime divisors of prime(n)*prime(n-1) - 1.

5, 9, 19, 21, 73, 18, 32, 111, 42, 451, 196, 381, 883, 108, 93, 526, 266, 232, 72, 2593, 36, 162, 1236, 98, 112, 752, 55, 26, 3081, 55, 4161, 1002, 9523, 135, 1616, 444, 863, 1368, 117, 415, 266, 3464, 2642, 1908, 1172, 3504, 1312, 2538, 135

Offset: 2

Author

Clark Kimberling

Keywords

• nonn
• look

Comments

If n-1 is in A103746, then a(n) = (prime(n)*prime(n-1)+3)/2. - Robert Israel, Jun 03 2020

Links

• Robert Israel, Table of n, a(n) for n = 2..10000

Crossrefs

Cf. A103746.

Programs

Maple

p:= 2: R:= NULL:
for n from 2 to 50 do
  q:= p; p:= nextprime(p);
  R:= R, convert(numtheory:-factorset(p*q-1),`+`)
od:
R; # Robert Israel, Jun 03 2020

PARI

sopf(n) = my(fac=factor(n)); sum(i=1, matsize(fac)[1], fac[i, 1]) ;
a(n) = sopf(prime(n)*prime(n-1) - 1); \\ Michel Marcus, Sep 30 2013

Formula

a(n) = A008472(A023515(n)). - Michel Marcus, Sep 30 2013

Extensions

Offset set to 2 and a(1) removed by Michel Marcus, Sep 30 2013