A091985 - OEIS

A091985

Number of steps required for the initial value p = 10^n to reach 0 in the recurrence p = pi(p).

1, 4, 6, 8, 9, 10, 11, 12, 12, 13, 14, 15, 15, 16, 17, 17, 18, 19, 19, 20

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OFFSET

0,2

LINKS

FORMULA

pi(n) = A000720(n) = number of primes less than or equal to n. By repeating n=pi(n), n will reach 0 in a finite number of steps.

EXAMPLE

Pi(100) = 25

Pi(25) = 9

Pi(9) = 4

Pi(4) = 2

Pi(2) = 1

Pi(1) = 0

Total steps to reach 0 = 6. Thus 6 is the 3rd entry in the sequence corresponding to n=2.

PROG

(PARI) pr10n(n) = my(c); for(x=0, n, y=10^x; c=0; p=y; while(p, p = primepi(p); c++); print1(c", "))

CROSSREFS

KEYWORD

hard,nonn

AUTHOR

EXTENSIONS

a(14)-a(19) from Chai Wah Wu, May 25 2018

STATUS

approved