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A121051
Semiprimes which are sums of 4 but no fewer nonzero squares.
0
15, 39, 55, 87, 95, 111, 119, 143, 159, 183, 215, 247, 287, 295, 303, 319, 327, 335, 391, 407, 415, 447, 471, 511, 519, 527, 535, 543, 551, 559, 583, 591, 623, 655, 671, 679, 687, 695, 703, 767, 791, 799, 807, 815, 831, 871, 879, 895, 943, 951, 959, 1007
OFFSET
1,1
COMMENTS
Semiprime analog of A007522 Primes of form 8n+7. These semiprimes must all be numbers of the form 4^i(8j+7), i >= 0, j >= 0. However, for positive i, 4^i(8j+7) has more than 2 prime factors (with multiplicity). Hence from Legendre's corollary to Lagrange's Four-Square Theorem, this sequence is precisely Semiprimes of the form 8*k+7.
MATHEMATICA
Select[8*Range[200]+7, PrimeOmega[#]==2&] (* Harvey P. Dale, Oct 28 2017 *)
CROSSREFS
A001358 intersect A004215.
Sequence in context: A336560 A176257 A055131 * A139042 A020142 A146696
KEYWORD
easy,nonn,less
AUTHOR
Jonathan Vos Post, Aug 08 2006
EXTENSIONS
a(15)-a(52) from Giovanni Resta, Jun 13 2016
STATUS
approved