This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
Join[{2,3},Table[SelectFirst[Join[10FromDigits[PadRight[{},k,3]]+{1,7,9},Flatten[Table[100 FromDigits[PadRight[{},k,3]]+10n+{1,3,7,9},{n,0,9}]],Flatten[Table[1000 FromDigits[PadRight[{},k,3]]+100n+{1,3,7,9},{n,0,99}]]],PrimeQ],{k,2,20}]] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Jun 01 2019 *)
A068103(n)={n=10^n\9*2;n>2&for(d=1,9e9,n*=10;for(t=1,10^d-1,ispseudoprime(n+t)&return(n+t)));2} \\ - M. F. Hasler, Oct 17 2012
from sympy import isprime
def a(n):
if n < 2: return list([2, 5])[n]
n5s, i, pow10, end_digits = int('5'*n), 1, 1, 0
while True:
i = 1
while i < pow10:
istr = str(i)
if istr[0] == '5' and len(istr) == end_digits:
i += 2 if pow10 <= 10 else pow10 // 10
else:
t = n5s * pow10 + i
if isprime(t): return t
i += 2
pow10 *= 10; end_digits += 1
print([a(n) for n in range(20)]) # Michael S. Branicky, Feb 05 2021, corrected Mar 03 2021
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