login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A343145 a(n) is the least positive number k such that applying x->prime(x) n times results in a number that ends in k. 1
1, 7, 6447, 7, 1, 1, 69, 9, 1, 1, 1, 7, 1, 1 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

LINKS

Table of n, a(n) for n=0..13.

EXAMPLE

a(1) = 7 since prime(7) = 17 which ends in 7.

a(2) = 6447 since prime(prime(6447)) = 806447 which ends in 6447.

a(3) = 7 since prime(prime(prime(7)))=277 which ends in 7.

a(4) = 1 since prime(prime(prime(prime(1)))) = 11 which ends in 1.

a(5) = 1 since prime(prime(prime(prime(prime(1))))) = 31 which ends in 1.

a(6) = 69 since prime(prime(prime(prime(prime(prime(69)))))) = 54615469 which ends in 69.

a(7) = 9 since prime(prime(prime(prime(prime(prime(prime(9))))))) = 4535189 which ends in 9.

PROG

(Python)

def A343145(n):

    k = 1

    while True:

        m = k

        for _ in range(n):

            m = prime(m)

        if m % 10**(len(str(k))) == k:

            return k

        k += 1

        while not (k % 2 and k % 5):

            k += 1

(PARI) primemap(n, tms) = my(x=n); for(i=1, tms, x=prime(x)); x

enddigits(n, len) = ((n/10^len - floor(n/10^len)) * 10^len)

a(n) = for(k=1, oo, my(x=k); if(enddigits(primemap(k, n), #Str(k))==k, return(k))) \\ Felix Fröhlich, Apr 14 2021

CROSSREFS

Cf. A046883, A074978, A343128.

Sequence in context: A246853 A225067 A281358 * A068575 A158400 A221628

Adjacent sequences:  A343142 A343143 A343144 * A343146 A343147 A343148

KEYWORD

nonn,hard,base,more

AUTHOR

Chai Wah Wu, Apr 06 2021

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified September 21 15:47 EDT 2021. Contains 347598 sequences. (Running on oeis4.)