OFFSET
1,1
COMMENTS
The first occurrence of a prime p in A039649 is not interesting because for an odd prime p it is evidently p.
Since phi(p) = phi(2p) = p-1 for odd prime p, then for n > 1 we have prime(n) < a(n) <= 2*prime(n).
For n > 1, a(n) is the smallest composite k such that phi(k) = prime(n)-1. - Thomas Ordowski, Jan 02 2017
If prime(n) > 7 is in A005385, then a(n) = 2*prime(n). - Thomas Ordowski (conjecture) and Robert Israel (proof), Jan 04 2017
LINKS
Michael De Vlieger, Table of n, a(n) for n = 1..10000
MATHEMATICA
Table[Function[p, First@ Drop[Lookup[#, p], 1]]@ Prime@ n, {n, 57}] &@ PositionIndex@ Table[EulerPhi@ n + 1, {n, 10^5}] (* Michael De Vlieger, Jan 02 2017, Version 10 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Vladimir Shevelev, May 10 2008
EXTENSIONS
Corrected and extended by Ray Chandler, May 20 2008
STATUS
approved