OFFSET
1,1
EXAMPLE
The sequence of terms together with their prime indices begins:
21: {2,4} 235: {3,15} 393: {2,32}
39: {2,6} 237: {2,22} 417: {2,34}
57: {2,8} 247: {6,8} 427: {4,18}
65: {3,6} 259: {4,12} 445: {3,24}
87: {2,10} 267: {2,24} 453: {2,36}
91: {4,6} 299: {6,9} 481: {6,12}
111: {2,12} 301: {4,14} 489: {2,38}
115: {3,9} 303: {2,26} 497: {4,20}
129: {2,14} 305: {3,18} 515: {3,27}
133: {4,8} 319: {5,10} 517: {5,15}
159: {2,16} 321: {2,28} 519: {2,40}
183: {2,18} 339: {2,30} 543: {2,42}
185: {3,12} 365: {3,21} 551: {8,10}
203: {4,10} 371: {4,16} 553: {4,22}
213: {2,20} 377: {6,10} 559: {6,14}
MATHEMATICA
Select[Range[100], SquareFreeQ[#]&&PrimeOmega[#]==2&&GCD@@PrimePi/@First/@FactorInteger[#]>1&]
CROSSREFS
A338909 is the not necessarily squarefree version.
A005117 lists squarefree numbers.
A339005 lists products of pairs of distinct primes of divisible index.
A320656 counts factorizations into squarefree semiprimes.
A338898, A338912, and A338913 give the prime indices of semiprimes, with product A087794, sum A176504, and difference A176506.
A338899, A270650, and A270652 give the prime indices of squarefree semiprimes, with difference A338900.
KEYWORD
nonn
AUTHOR
Gus Wiseman, Nov 22 2020
STATUS
approved