OFFSET
1,1
COMMENTS
1153 and 1381 are also terms. - Amiram Eldar, Feb 16 2020
a(20) >= 1091. 1091# - 1 is a 458-digit composite with no known factors. - Hugo Pfoertner, Feb 05 2021
LINKS
FactorDB Factorization of 1013#-1.
FactorDB Status of 1091#-1.
Hisanori Mishima, Factorizations of many number sequences, PI Pn - 1 (n = 1 to 110) (2013).
FORMULA
EXAMPLE
a(2) = 17 so 17# - 1 = 510509 = 61 * 8369 is a semiprime.
MATHEMATICA
p = Select[Range[101], PrimeQ]; p[[ Position[FoldList[Times, p] - 1, _?(PrimeOmega[#] == 2 &)] //Flatten ]] (* Amiram Eldar, Feb 16 2020 *)
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Eric Chen, Dec 24 2014
EXTENSIONS
a(15)-a(18) using factordb.com from Amiram Eldar, Feb 16 2020
a(19) using factordb.com from Hugo Pfoertner, Feb 05 2021
Edited by Max Alekseyev, Aug 26 2021
STATUS
approved