Let p > 3 p>3 be a prime. Euler numbers E p − 3 E_{p-3} first appeared in H. S. Vandiver’s work (1940) in connection with the first case of Fermat’s Last Theorem. Vandiver proved that if x p + y p = z p x^p+y^p=z^p has a solution for integers x , y , z x,y,z with gcd ( x y z , p ) = 1 \gcd (xyz,p)=1 , then it must be that E p − 3 ≡ 0 ( mod p ) E_{p-3}\equiv 0\,(\bmod \,p) . Numerous combinatorial congruences recently obtained by Z.-W. Sun and Z.-H. Sun involve the Euler numbers E p − 3 E_{p-3} . This gives a new significance to the primes p p for which E p − 3 ≡ 0 ( mod p ) E_{p-3}\equiv 0\,(\bmod \,p) . For the computation of residues of Euler numbers E p − 3 E_{p-3} modulo a prime p p , we use a congruence which runs significantly faster than other known congruences involving E p − 3 E_{p-3} . Applying this, congruence, via a computation in Mathematica 8, shows that there are only three primes less than 10 7 10^7 that satisfy the condition E p − 3 ≡ 0 ( mod p ) E_{p-3}\equiv 0\,(\bmod \,p) (these primes are 149, 241 and 2946901). By using related computational results and statistical considerations similar to those used for Wieferich, Fibonacci-Wieferich and Wolstenholme primes, we conjecture that there are infinitely many primes p p such that E p − 3 ≡ 0 ( mod p ) E_{p-3}\equiv 0\,(\bmod \,p) .
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