A101593 a(n) is the number of m <= 2^n which are in A075190, i.e., such that m^2 is exactly at the center between two consecutive primes, or in other words A056929(m) = 0.

1, 2, 3, 6, 9, 14, 19, 34, 62, 109, 202, 336, 587, 1100, 2003, 3630, 6784, 12607, 23647, 44206, 83510, 157851, 299810, 571264, 1090986, 2088445, 4004347, 7687694, 14788984, 28496850, 54955214, 106159961

Offset

1,2

Comments

A056929(m) = 0 iff m^2 is an interprime <=> m^2 = (A007491(m^2) + A053001(m^2))/2 = average of the next higher and next lower primes.

From M. F. Hasler, Oct 18 2022: (Start)

The ratio a(n+1)/a(n) oscillates between 1.5 and 2 for the first few values, but then seems to converge to a limit between 1.9 and 2; from n = 19 on these ratios appear to be strictly increasing (from 1.87 at n = 19 to 1.92 at n = 27).

At first sight it seems natural that there are twice as many interprimes of the form f(m) when the upper limit on m is increased by a factor of 2, but this might depend on the function f.

If instead of m^2 we consider the same for m^3, then we find the sequence 0, 1, 1, 3, 5, 8, 18, 29, 52, 86, 136, 223, 421, 758, 1376, 2517, 4616, 8714, 16173, 30414, 57583, 109539, ... which follows roughly the same asymptotic behavior. (End)

Links

Table of n, a(n) for n=1..32.

Formula

a(1) counts the squares m^2 with m <= 2^n = 2 which are interprimes. The squares 0^2 = 0 and 1^2 = 1 obviously aren't interprimes, so the only such square in that range is m^2 = 2^2 = 4 = (nextprime + precprime)/2 = (3 + 5)/2, so a(1) = 1.

Then for n = 2, up to m <= 2^n = 4 we have the additional squares m^2 = 3^2 = 9 = (7 + 11)/2 (an interprime) and m^2 = 4^2 = 16 <> (13 + 17)/2 = 15, so this m^2 is not an interprime, and a(2) = a(1) + 1 = 2.

Prog

(PARI) a(n)=sum(i=2, 2^n, !A056929(i))
(PARI) a(n)=sum(i=2, 2^n, nextprime(i^2)+precprime(i^2)==2*i^2)
(PARI)
t=0; vector(15, n, t+=sum(i=1/2<<n+1, 1<<n, nextprime(i^2)+precprime(i^2)==2*i^2))
/* Then, to compute progressively more values: (uses the variable t set to a(15) by the previous command; to continue the computation, set t to the last value printed and restart at the next index. [Explanation added by M. F. Hasler, Oct 18 2022] */
for(n=16, 30, print1("/* a("n") = */ ", t += sum(i=2^(n-1)+1, 2^n, nextprime(i^2)+precprime(i^2)==2*i^2), ", "))

Crossrefs

Cf. A007491, A053001, A056929, A069495, A075190.

Sequence in context: A261243 A061925 A073736 * A349502 A226893 A084628

Adjacent sequences: A101590 A101591 A101592 * A101594 A101595 A101596

Keyword

nonn,more

Author

Zak Seidov and M. F. Hasler, Dec 27 2007

Extensions

a(23)-a(25) from Kevin P. Thompson, Nov 26 2021

a(26)-a(28) from M. F. Hasler, Oct 18 2022

a(29)-a(32) from Bill McEachen, Dec 14 2022

Status

approved