A087633 Number of n-digit primes ending in 9 in base 10.

0, 5, 33, 265, 2087, 17203, 146439, 1274154, 11271147, 101049993, 915748570, 8372464236, 77114396969

Offset: 1

Author

Meenakshi Srikanth (menakan_s(AT)yahoo.com) and Amarnath Murthy, Sep 15 2003

Keywords

• nonn
• base
• hard
• more

Examples

			a(2) = 5 as there exist 5 two-digit prime numbers (19, 29, 59, 79, and 89) with units place 9.
a(3) = 33, since there are 33 three-digit numbers with units place digit as 9.
		

Crossrefs

Cf. A006879, A073508, A087630, A087631, A087632.

Programs

Java

/** The terms of the sequences are generated by changing the range for j for the various numbers of digits. E.g., it ranges from 100 to 999 for three-digit numbers. */
float r, x;
int c = 0, count = 0; for (float j = 100f; j < 1000f; j++) { for (float i = 2f; i < j; i++) { r = j % i; if (r == 0) c = 1; } if (c == 0) { x = j % 10; if (x == 9) count = count + 1; } c = 0; } System.out.println("count = " + count);

Mathematica

Table[Length[Select[Range[10^n + 9, 10^(n + 1) - 1, 10], PrimeQ]], {n, 5}] (* Alonso del Arte, Apr 27 2014 *)

PARI

a(n) = my(c=0); forprime(p=10^(n-1), 10^n, if(p%10==9, c++)); c \\ Iain Fox, Aug 07 2018

Formula

From Iain Fox, Aug 07 2018: (Start)

a(n) ~ (1/4) * Integral_{x=10^(n-1)..10^n} (dx/log(x)).

a(n) = A006879(n) - A087630(n) - A087631(n) - A087632(n), for n > 1.

(End)

Extensions

Corrected and extended by Ray Chandler, Oct 04 2003

Offset corrected by Iain Fox, Aug 07 2018

a(11) from Iain Fox, Aug 07 2018

a(12)-a(13) from Giovanni Resta, Aug 07 2018