OFFSET
3,1
COMMENTS
Compared to A001747, we have 21 instead of 22 and lack 2 and 4.
Compared to A100484 (shifted) we have 21 instead of 22 and lack 4.
Compared to A161344, we have 21 instead of 22 and lack 4 and 8.
Compared to A339114, we have 11 instead of 9 and lack 4.
A prime index of n is a number m such that prime(m) divides n. The multiset of prime indices of n is row n of A112798.
A squarefree semiprime (A006881) is a product of any two distinct prime numbers.
EXAMPLE
The initial terms and their prime indices:
6: {1,2}
10: {1,3}
14: {1,4}
21: {2,4}
26: {1,6}
34: {1,7}
38: {1,8}
46: {1,9}
MATHEMATICA
Table[Min@@Select[Table[Times@@Prime/@y, {y, IntegerPartitions[n, {2}]}], SquareFreeQ], {n, 3, 50}]
PROG
(Python)
from sympy import prime, sieve
def a(n):
p = [0] + list(sieve.primerange(1, prime(n)+1))
return min(p[i]*p[n-i] for i in range(1, (n+1)//2))
print([a(n) for n in range(3, 58)]) # Michael S. Branicky, Sep 05 2021
CROSSREFS
The opposite version (greatest instead of smallest) is A332765.
These are the minima of rows of A338905.
A246868 gives the greatest squarefree number whose prime indices sum to n.
A339362 adds up prime indices of squarefree semiprimes.
KEYWORD
nonn
AUTHOR
Gus Wiseman, Aug 22 2021
STATUS
approved