A334132 Smallest prime factor of n-th term in Look and Say sequence A005150, with a(1)=1.

1, 11, 3, 7, 11, 312211, 13112221, 11, 5581, 26966089, 7, 20328937, 29, 3, 3, 3, 1637, 103, 50593, 43, 13, 19, 17, 103, 31, 19, 7, 3, 19, 1208033, 23, 3, 3, 83, 3, 233, 3, 3

Offset

1,2

Comments

The terms in A005150 that are known to be primes are a(2) = 11, a(6) = 312211 and a(7) = 13112221 (A100108).

a(n) = 3 iff A004977(n) is positive and divisible by 3.

a(39) > 2*10^9. a(62) > 10^7. - Tyler Busby, Jan 25 2023

Links

Table of n, a(n) for n=1..38.

Tyler Busby, Table of n, a(n) for n = 1..75 with -1 for those entries where a(n) has not yet been found, Jan 25 2023.

Wikipedia, Look-and-say sequence

Formula

a(n) = A020639(A005150(n)).

Example

A005150(7) = 13112221 is prime and a(7) = 13112221.
A005150(9) = 31131211131221 = 5581 * 5578070441, hence a(9) = 5581.

Crossrefs

Cf. A004977 (sum of digits of terms of A005150), A005150 (Look and Say sequence), A020639, A079562, A100108 (primes in A005150).

Sequence in context: A071234 A082626 A344542 * A245193 A338715 A258190

Adjacent sequences: A334129 A334130 A334131 * A334133 A334134 A334135

Keyword

nonn,base,more

Author

Bernard Schott, Apr 15 2020

Extensions

a(17)-a(38) from Jinyuan Wang, Apr 15 2020

Status

approved