A260597 Primes as they occur for the first time as factors of terms of A173426 = concatenation(1,2,...,n,n-1,...,1).

11, 3, 37, 101, 41, 271, 7, 13, 239, 4649, 73, 137, 333667, 12345678910987654321, 17636684157301569664903, 2799473675762179389994681, 1109, 4729, 2354041513534224607850261, 571, 3167, 10723, 439781, 2068140300159522133, 75401, 687437, 759077450603

Offset: 1

Author

M. F. Hasler, Jul 29 2015

Keywords

• nonn

Comments

Or, distinct elements of A260589 in the order they occur for the first time.

Examples

			A173426(1) = 1;  A173426(2) = 121 = 11^2 => a(1) = 11.
A173426(3) = 12321 = 3^2 37^2 => a(2..3) = (3, 37).
A173426(4) = 1234321 = 11^2 101^2 => a(4) = 101.
A173426(5) = 123454321 = 41^2 271^2 => a(5..6) = (41, 271).
A173426(6) = 12345654321 = 3^2 7^2 11^2 13^2 37^2 => a(7..8) = (7, 13).
		

Links

• M. F. Hasler and Chai Wah Wu, Table of n, a(n) for n = 1..114 (a(n) for n = 1..84 from M. F. Hasler)

Programs

PARI

S=[];apply(t->S=setunion(S,t=setminus(Set(t),S));t, vector(30,n,A260589_row(n)))

Python

from sympy import primefactors
A260597_list = []
for n in range(1,10):
    m = primefactors(int(''.join([str(d) for d in range(1,n+1)]+[str(d) for d in range(n-1,0,-1)])))
    for p in m:
        if not p in A260597_list:
            A260597_list.append(p) # Chai Wah Wu, Aug 11 2015