cp's OEIS Frontend

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.

%I A343145 #18 Apr 20 2021 10:35:08
%S A343145 1,7,6447,7,1,1,69,9,1,1,1,7,1,1
%N 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.
%e A343145 a(1) = 7 since prime(7) = 17 which ends in 7.
%e A343145 a(2) = 6447 since prime(prime(6447)) = 806447 which ends in 6447.
%e A343145 a(3) = 7 since prime(prime(prime(7)))=277 which ends in 7.
%e A343145 a(4) = 1 since prime(prime(prime(prime(1)))) = 11 which ends in 1.
%e A343145 a(5) = 1 since prime(prime(prime(prime(prime(1))))) = 31 which ends in 1.
%e A343145 a(6) = 69 since prime(prime(prime(prime(prime(prime(69)))))) = 54615469 which ends in 69.
%e A343145 a(7) = 9 since prime(prime(prime(prime(prime(prime(prime(9))))))) = 4535189 which ends in 9.
%o A343145 (Python)
%o A343145 def A343145(n):
%o A343145     k = 1
%o A343145     while True:
%o A343145         m = k
%o A343145         for _ in range(n):
%o A343145             m = prime(m)
%o A343145         if m % 10**(len(str(k))) == k:
%o A343145             return k
%o A343145         k += 1
%o A343145         while not (k % 2 and k % 5):
%o A343145             k += 1
%o A343145 (PARI) primemap(n, tms) = my(x=n); for(i=1, tms, x=prime(x)); x
%o A343145 enddigits(n, len) = ((n/10^len - floor(n/10^len)) * 10^len)
%o A343145 a(n) = for(k=1, oo, my(x=k); if(enddigits(primemap(k, n), #Str(k))==k, return(k))) \\ _Felix Fröhlich_, Apr 14 2021
%Y A343145 Cf. A046883, A074978, A343128.
%K A343145 nonn,hard,base,more
%O A343145 0,2
%A A343145 _Chai Wah Wu_, Apr 06 2021