cp's OEIS Frontend

A091089 Numbers which form a prime by appending a 3-digit odd number and form no primes by appending any 1- or 2-digit odd number not beginning with 0.

%I A091089 #18 Jun 08 2025 16:15:42
%S A091089 16557,16718,26378,35921,46524,46867,50018,55187,58374,58452,60850,
%T A091089 63714,68771,71299,78035,78269,81661,84213,89052,90157,95490,97080,
%U A091089 102892,105690,108682,115558,115994,116138,116305,121097,128192,131194
%N A091089 Numbers which form a prime by appending a 3-digit odd number and form no primes by appending any 1- or 2-digit odd number not beginning with 0.
%C A091089 Many numbers become prime by appending a one-digit odd number. Some numbers (such as 20, 32, 51, etc.) require a 2-digit odd number (A032352 has these). In the first 100000 values of n there are only 22 that require a 3-digit odd number.
%H A091089 Robert Price, <a href="/A091089/b091089.txt">Table of n, a(n) for n = 1..23419</a>
%H A091089 <a href="/index/Pri#piden">Index entries for primes involving decimal expansion of n</a>
%e A091089 a(1)=16557 because 16557 is first number which requires a 3-digit odd number be appended to it to form a prime. 165571, 165573, 165575, ..., 165579, 1655711, 1655713, ..., 1655799 are all nonprime numbers. 16557103 is the first prime formed by appending odd numbers to 16657.
%e A091089 a(2) = 16718 because 16718111 is the first prime formed by appending odd numbers to 16718.
%Y A091089 Cf. A032352 (a(n) requires at least a 2-digit odd number), A068695 (minimum odd number that must be appended to n to form a prime).
%K A091089 base,nonn
%O A091089 1,1
%A A091089 _Chuck Seggelin_, Dec 18 2003
%E A091089 Definition edited by _N. J. A. Sloane_, Nov 08 2020