OFFSET
1,2
COMMENTS
Also the least k such that the n-th primorial (A002110) is a divisor of the k-th compositorial (A036691). - Reinhard Zumkeller, Sep 03 2002
LINKS
Harry J. Smith, Table of n, a(n) for n = 1..1000
FORMULA
a(n) = 2*prime(n) - (pi(2*prime(n))) - 1, where pi = A000720.
EXAMPLE
a(7) = 22 because twice the 7th prime (2*17 = 34) is the 22nd composite number: 4, 6, 8, 9, 10, 12, 14, 15, 16, 18, 20, 21, 22, 24, 25, 26, 27, 28, 30, 32, 33, 34.
MAPLE
A065897:=n->2*ithprime(n)-(numtheory[pi](2*ithprime(n)))-1: seq(A065897(n), n=1..100); # Wesley Ivan Hurt, Sep 16 2017
MATHEMATICA
Table[2*Prime[n]-(PrimePi[2*Prime[n]])-1, {n, 128}]
PROG
(PARI) { for (n=1, 1000, f=2*prime(n); a=f - primepi(f) - 1; write("b065897.txt", n, " ", a) ) } \\ Harry J. Smith, Nov 04 2009
(Magma)
A065897:= func< n | 2*NthPrime(n) -1 -#PrimesUpTo(2*NthPrime(n)) >;
[A065897(n): n in [1..130]]; // G. C. Greubel, Aug 24 2024
(SageMath)
def A065897(n): return 2*nth_prime(n) -prime_pi(2*nth_prime(n)) -1
[A065897(n) for n in range(1, 131)] # G. C. Greubel, Aug 24 2024
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Labos Elemer, Nov 28 2001
STATUS
approved