[seqfan] Semiprimes in A187787 and related issues

[Tomasz Ordowski]

Dear SeqFans,

I have an interesting question:

Semiprimes n = pq such that 2^(p+q) == 1 (mod n)
are semiprimes n such that 2^(n+1) == 1 (mod n) ?

This is confirmed by the data from Amiram Eldar:
15, 35, 119, 5543, 74447, 90859, 110767, 222179,
389993, 1526849, 2927297, 3626699, 4559939,
24017531, 137051711, 160832099, 229731743, ...
These are semiprimes in A187787.

By the way, I will also ask:

Semiprimes n such that lambda(n) | (n+1).
I only see two such numbers: 15 and 35.
How to prove that there are no others?
These are semiprimes in A208728.

Note that {A208728} is a subset of {A187787}.
While {A203966 > 3} is a subset of {A208728}.
Has it been proved that this last set is infinite?

Best regards,

Thomas Ordowski
____________________
https://oeis.org/A187787
https://oeis.org/A208728
https://oeis.org/A203966