A343158 - OEIS

%I #27 Jul 17 2022 02:22:38

%S 2,4,10,35,15,34,190,290,303,395,130,465,553,265,195,663,218,582,481,

%T 858,714,418,345,530,382,1771,1207,2098,3890,1426,2090,4834,4618,627,

%U 2321,2163,326,866,3302,1298,3886,3094,1086,6130,4807,3646,5181,905,3945,5753

%N a(n) is the smallest m such that A343156(m) = n, or -1 if no such m exists.

%D Eric Angelini, W. Edwin Clark, Hans Havermann, Frank Stevenson, Allan C. Wechsler, and others, Postings to Math Fun mailing list, April 2021.

%e 2 takes 0 steps to reach a prime, so a(0) = 2.

%e 10 -> 25 -> 5 takes 2 steps to reach a prime (and no smaller number takes that many steps), so a(2) = 10.

%e 35 -> 57 -> 319 -> 1129 takes 3 steps to reach a prime (and no smaller number takes that many steps), so a(3) = 35.

%o (PARI) is(m, n) = my(k=m); for(i=1, n, if(isprime(k), return(0), k=eval(concat(apply(t->Str(t), factor(k)[, 1]~))))); isprime(k);

%o a(n) = for(m=2, oo, if(is(m, n), return(m))); \\ _Jinyuan Wang_, Jul 16 2022

%Y Cf. A084317, A084319, A343156.

%K nonn,base

%O 0,1

%A _N. J. A. Sloane_, Apr 07 2021

%E a(32)-a(42) from _Hans Havermann_, Apr 07 2021

%E a(43)-a(48) from _Hans Havermann_, Apr 08 2021

%E a(49) from _Jinyuan Wang_, Jul 16 2022