A359490 Primes p followed by two or more 2-pseudoprimes (A001567) before the next prime.

4363, 13729, 31607, 6973007, 208969199

Offset: 1

Author

Charles R Greathouse IV, Mar 07 2023

Keywords

• nonn
• more

Comments

Is this sequence infinite? Is there a prime followed by three pseudoprimes before the next prime? I believe heuristics suggest "no" to both.

From Amiram Eldar, Mar 12 2023: (Start)

The primes preceding the terms of A335326.

There are no more terms below 2^64. (End)

Examples

			a(n): pseudoprimes following a(n)
4363: 4369, 4371
13729: 13741, 13747
31607: 31609, 31621
6973007: 6973057, 6973063
208969199: 208969201, 208969223
		

Crossrefs

Cf. A001567, A335326.

Programs

PARI

prp(n,a=2)=Mod(a,n)^(n-1)==1
list(lim)=my(v=List(),p=3); forprime(q=5,lim, my(s=0); forstep(k=p+2,q-2,2, if(prp(k) && s++>1, listput(v,p))); p=q); Vec(v)