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%I #13 Jan 10 2022 15:43:45
%S 11,3,37,101,41,271,7,13,239,4649,73,137,333667,12345678910987654321,
%T 17636684157301569664903,2799473675762179389994681,1109,4729,
%U 2354041513534224607850261,571,3167,10723,439781,2068140300159522133,75401,687437,759077450603
%N Primes as they occur for the first time as factors of terms of A173426 = concatenation(1,2,...,n,n-1,...,1).
%C Or, distinct elements of A260589 in the order they occur for the first time.
%H M. F. Hasler and Chai Wah Wu, <a href="/A260597/b260597.txt">Table of n, a(n) for n = 1..114</a> (a(n) for n = 1..84 from M. F. Hasler)
%e A173426(1) = 1; A173426(2) = 121 = 11^2 => a(1) = 11.
%e A173426(3) = 12321 = 3^2 37^2 => a(2..3) = (3, 37).
%e A173426(4) = 1234321 = 11^2 101^2 => a(4) = 101.
%e A173426(5) = 123454321 = 41^2 271^2 => a(5..6) = (41, 271).
%e A173426(6) = 12345654321 = 3^2 7^2 11^2 13^2 37^2 => a(7..8) = (7, 13).
%o (PARI) S=[];apply(t->S=setunion(S,t=setminus(Set(t),S));t, vector(30,n,A260589_row(n)))
%o (Python)
%o from sympy import primefactors
%o A260597_list = []
%o for n in range(1,10):
%o m = primefactors(int(''.join([str(d) for d in range(1,n+1)]+[str(d) for d in range(n-1,0,-1)])))
%o for p in m:
%o if not p in A260597_list:
%o A260597_list.append(p) # _Chai Wah Wu_, Aug 11 2015
%K nonn
%O 1,1
%A _M. F. Hasler_, Jul 29 2015
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