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A080693 Numbers of the form p^2*q + r*s where p,q,r,s are (not necessarily distinct) primes. 0
12, 14, 16, 17, 18, 21, 22, 23, 24, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

A conjecture of Goldbach type says every number >= 26 is of this form.

LINKS

Table of n, a(n) for n=1..69.

EXAMPLE

12=2^2*2 + 2*2

MAPLE

H := proc(n::posint) local p, q, r, s; p := 2; while p<=floor(sqrt((n-4)/2)) do q := 2; while q<=floor((n-4)/p^2) do s := 2; while s<=floor((n-p^2*q)/2) do r := (n-p^2*q)/s; if type(r, posint) then if isprime(r) then return(true, p, q, s, r); end if; end if; s := nextprime(s); end do; q := nextprime(q); end do; p := nextprime(p); end do; return(false); end:

MATHEMATICA

Take[ Union[ Flatten[ Table[ Prime[p]^2*Prime[q] + Prime[r]*Prime[s], {p, 1, 6}, {q, 1, 15}, {r, 1, 15}, {s, 1, 15}]]], 70]

CROSSREFS

Cf. A081053.

Sequence in context: A043651 A043701 A290001 * A135739 A096923 A141642

Adjacent sequences:  A080690 A080691 A080692 * A080694 A080695 A080696

KEYWORD

nonn

AUTHOR

Mario Maqueda Garcia [Garci'a] (israelmira(AT)terra.es), Mar 03 2003

EXTENSIONS

Edited and extended by Robert G. Wilson v, Mar 05 2003

STATUS

approved

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Last modified December 8 08:49 EST 2019. Contains 329862 sequences. (Running on oeis4.)