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A290435 Semiprimes of the form pq where p < q and p+q+1 is prime. 2
21, 35, 39, 55, 57, 65, 77, 85, 111, 115, 129, 155, 161, 185, 187, 201, 203, 205, 209, 221, 235, 237, 265, 291, 299, 305, 309, 319, 323, 327, 335, 341, 365, 371, 377, 381, 391, 413, 415, 437, 451, 485, 489, 493, 497, 505, 515, 517, 535, 579, 611, 623, 649, 655 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Squarefree terms of A290434.

All terms are odd.

A286842(a(n)) = 1 for all n.

LINKS

Chai Wah Wu, Table of n, a(n) for n = 1..10000

EXAMPLE

655 = 5*131 and 5+131+1 is prime, so 655 is a term.

MATHEMATICA

With[{nn = 54}, Take[#, nn] &@ Union@ Flatten@ Table[Function[p, Map[Times @@ # &@ # &, #] &@ Select[Map[{p, #} &, Prime@ Range[PrimePi@ p - 1]], PrimeQ[Total@ # + 1] &]]@ Prime@ n, {n, nn + 4}]] (* Michael De Vlieger, Aug 01 2017 *)

With[{nn=60}, Take[Times@@@Select[Subsets[Prime[Range[nn]], {2}], PrimeQ[ Total[ #]+ 1]&]//Union, nn]] (* Harvey P. Dale, Aug 02 2017 *)

PROG

(Python)

from sympy import factorint, isprime

A290435_list = [n for n in range(2, 10**5) if sum(factorint(n).values()) == len(factorint(n)) == 2 and isprime(1+sum(factorint(n).keys()))]

(PARI) isok(n) = (bigomega(n)==2) && (omega(n)==2) && isprime(1+vecsum(factor(n)[, 1])); \\ Michel Marcus, Aug 02 2017

CROSSREFS

Cf. A001358, A005117, A006881, A286842, A290434.

Sequence in context: A216467 A330949 A248020 * A138227 A246157 A301789

Adjacent sequences:  A290432 A290433 A290434 * A290436 A290437 A290438

KEYWORD

nonn

AUTHOR

Chai Wah Wu, Aug 01 2017

STATUS

approved

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Last modified March 30 16:16 EDT 2020. Contains 333127 sequences. (Running on oeis4.)