1,3

Table of n, a(n) for n=1..9.

19 is prime. ==> a(1) = 0.

991 is prime. ==> a(2) = 1.

90901 is prime. ==> a(3) = 2.

9009001 = 131 * 68771.

900090001 = 421 * 2137981.

90000900001 = 131 * 701 * 980071.

9000009000001 is prime. ==> a(4) = 6.

(PARI) for(k=0, 1e3, if(ispseudoprime(9*100^k+9*10^k+1), print1(k", ")))

Cf. A309739.

Sequence in context: A179201 A105122 A132076 * A291062 A058046 A298410

Adjacent sequences: A309740 A309741 A309742 * A309744 A309745 A309746

nonn,more

Seiichi Manyama, Aug 15 2019

approved