A176707 Sum of digits of all distinct prime factors of n-th semiprime.

2, 5, 3, 7, 9, 8, 10, 4, 5, 6, 5, 10, 12, 12, 7, 7, 7, 11, 7, 13, 13, 6, 9, 8, 12, 9, 7, 13, 9, 14, 11, 7, 13, 15, 10, 13, 10, 16, 15, 2, 9, 8, 10, 17, 15, 14, 10, 6, 16, 12, 9, 18, 11, 12, 13, 4, 17, 19, 10, 15, 10, 18, 16, 4, 18, 10, 6, 12, 11, 10, 12, 11, 12, 13, 12, 7, 16, 19, 14, 14, 7

Offset: 1

Author

Juri-Stepan Gerasimov, Apr 24 2010

Keywords

• nonn
• base

Examples

			a(1)=2 because 1st semiprime=2*2 and 2=2; a(2)=5 because 2nd semiprime=2*3 and 2<3.
		

Links

• Michael S. Branicky, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A095402.

Programs

Maple

A007953 := proc(n) add(d,d=convert(n,base,10)) ; end proc:
A176707 := proc(n) s := A001358(n) ; add( A007953(p), p = numtheory[factorset](s) ) ; end proc: seq(A176707(n),n=1..120) ; # R. J. Mathar, Apr 25 2010

Python

from sympy import factorint
def aupton(terms):
  alst, m = [], 4
  while len(alst) < terms:
    f = factorint(m)
    if sum(f.values()) == 2: # semiprime
      alst.append(sum(sum(map(int, str(p))) for p in f.keys()))
    m += 1
  return alst
print(aupton(81)) # Michael S. Branicky, Feb 05 2021

Extensions

a(13), a(34) etc. corrected by - R. J. Mathar, Apr 25 2010