A127329 Semiprimes equal to the sum of three primes in arithmetic progression.

15, 21, 33, 39, 51, 57, 69, 87, 93, 111, 123, 129, 141, 159, 177, 183, 201, 213, 219, 237, 249, 267, 291, 303, 309, 321, 327, 339, 381, 393, 411, 417, 447, 453, 471, 489, 501, 519, 537, 543, 573, 579, 591, 597, 633, 669, 681, 687, 699, 717, 723, 753, 771, 789

Offset: 1

Author

Giovanni Teofilatto, Mar 30 2007

Keywords

• easy
• nonn

Examples

			a(1) = 15 because 15 = 3 + 5 + 7;
a(2) = 21 because 21 = 3 + 7 + 11.
		

Crossrefs

Cf. A000040, A001358, A078611.

Programs

Magma

[3*NthPrime(n+2): n in [1..60]]; // Vincenzo Librandi, Jun 28 2015, assuming the conjecture holds

Formula

Conjecture: a(n) = 3*A000040(n+2). - Zak Seidov, Jun 28 2015

Every member of the sequence is 3 times a prime; it is believed that every prime >= 5 arises in this way. This is related to Goldbach's conjecture: see comments to A078611. - Robert Israel and Michel Marcus, Jun 28 2015

Extensions

Corrected (57 = 7+19+31, 87 = 5+29+53, etc. inserted) by R. J. Mathar, Apr 22 2010