A343158 a(n) is the smallest m such that A343156(m) = n, or -1 if no such m exists.

2, 4, 10, 35, 15, 34, 190, 290, 303, 395, 130, 465, 553, 265, 195, 663, 218, 582, 481, 858, 714, 418, 345, 530, 382, 1771, 1207, 2098, 3890, 1426, 2090, 4834, 4618, 627, 2321, 2163, 326, 866, 3302, 1298, 3886, 3094, 1086, 6130, 4807, 3646, 5181, 905, 3945, 5753

Offset

0,1

References

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

Links

Table of n, a(n) for n=0..49.

Example

2 takes 0 steps to reach a prime, so a(0) = 2.
10 -> 25 -> 5 takes 2 steps to reach a prime (and no smaller number takes that many steps), so a(2) = 10.
35 -> 57 -> 319 -> 1129 takes 3 steps to reach a prime (and no smaller number takes that many steps), so a(3) = 35.

Prog

(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);
a(n) = for(m=2, oo, if(is(m, n), return(m))); \\ Jinyuan Wang, Jul 16 2022

Crossrefs

Cf. A084317, A084319, A343156.

Sequence in context: A109455 A372495 A258948 * A189591 A189598 A156800

Adjacent sequences: A343155 A343156 A343157 * A343159 A343160 A343161

Keyword

nonn,base

Author

N. J. A. Sloane, Apr 07 2021

Extensions

a(32)-a(42) from Hans Havermann, Apr 07 2021

a(43)-a(48) from Hans Havermann, Apr 08 2021

a(49) from Jinyuan Wang, Jul 16 2022

Status

approved