The OEIS mourns the passing of Jim Simons and is grateful to the Simons Foundation for its support of research in many branches of science, including the OEIS.
The OEIS is supported by the many generous donors to the OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A271550 Numbers n such that n is a squarefree semiprime (i.e., omega(n) = 2 = Omega(n)) and phi(n) + 1 is a prime. 1
 6, 10, 14, 21, 22, 26, 34, 38, 46, 55, 57, 58, 62, 74, 77, 82, 86, 91, 93, 94, 95, 106, 111, 115, 118, 119, 122, 133, 134, 142, 145, 146, 158, 166, 178, 194, 202, 206, 209, 214, 217, 218, 221, 226, 237, 254, 262, 274, 278, 287, 291, 295, 298, 302, 305, 314, 319, 326, 329 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Equals (A001358 intersection A039698) - A001248. LINKS Harvey P. Dale, Table of n, a(n) for n = 1..1000 EXAMPLE 21 is in the sequence, because 21 = 3*7 is a semiprime with omega(21) = 2 and phi(21) + 1 = 2*6 + 1 = 13 is a prime. 55 is in the sequence, because 55 = 5*11 is a semiprime with omega(55) = 2 and phi(55) + 1 = 4*10 + 1 = 41 is a prime. MATHEMATICA Select[Range[400], SquareFreeQ[#]&&PrimeOmega[#]==2&&PrimeQ[EulerPhi[ #]+ 1]&] (* Harvey P. Dale, Aug 08 2020 *) PROG (PARI) is(n)=my(f=factor(n)); f[, 2]==[1, 1]~ && isprime((f[1, 1]-1)*(f[2, 1]-1)+1) \\ Charles R Greathouse IV, Jul 21 2016 CROSSREFS Cf. A001358, A039698, A001248. Sequence in context: A068198 A064899 A339005 * A315224 A315225 A068014 Adjacent sequences: A271547 A271548 A271549 * A271551 A271552 A271553 KEYWORD nonn AUTHOR Debapriyay Mukhopadhyay, Jul 13 2016 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified May 18 18:37 EDT 2024. Contains 372664 sequences. (Running on oeis4.)