1,2

Table of n, a(n) for n=1..65.

For n = 4 we have prime(n) = prime(4) = 7. The least k such that k*prime(4) is practical is k = 4, to get 28. Therefore, a(4) = 4.

(PARI) a(n) = my(p = prime(n)); for(i = 1, oo, if(is_A005153(p * i), return(i))) \\ David A. Corneth, Dec 31 2018

Cf. A005153.

Sequence in context: A135974 A153494 A205404 * A316945 A307370 A014246

Adjacent sequences: A322369 A322370 A322371 * A322373 A322374 A322375

nonn

David A. Corneth, Dec 05 2018

approved