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URL: https://oeis.org/A075320

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A075320
Pair the odd numbers such that the k-th pair is (r, r+2k) where r is the smallest odd number not included earlier: (1, 3), (5, 9), (7, 13), (11, 19), (15, 25), (17, 29), (21, 35), (23, 39), (27, 45), ... This is the sequence of the product of the members of pairs.
4
3, 45, 91, 209, 375, 493, 735, 897, 1215, 1581, 1815, 2257, 2747, 3053, 3619, 3969, 4611, 5301, 5723, 6489, 6955, 7797, 8687, 9225, 10191, 11205, 11815, 12905, 13559, 14725, 15939, 16665, 17955, 19293, 20091, 21505, 22347, 23837, 25375, 26289
OFFSET
1,1
FORMULA
a(n) = A075317(n)*A075318(n). - R. J. Mathar, Nov 12 2006
MAPLE
A075320 := proc(nmax) local r, k, a, pairs ; a := [3] ; pairs := [1, 3] ; k := 2 ; r := 5 ; while nops(a) < nmax do while r in pairs do r := r+2 ; od ; if r+2*k in pairs then printf("inconsistency", k) ; fi ; a := [op(a), r*(r+2*k)] ; pairs := [op(pairs), r, r+2*k] ; k := k+1 ; od ; RETURN(a) ; end: a := A075320(200) : for n from 1 to nops(a) do printf("%d, ", op(n, a)) ; od ; # R. J. Mathar, Nov 12 2006
PROG
(Python)
from math import isqrt
def A075320(n): return (m:=(n+isqrt(5*n**2)&-2)-1)*((n<<1)+m) # Chai Wah Wu, Aug 16 2022
CROSSREFS
KEYWORD
nonn
AUTHOR
Amarnath Murthy, Sep 14 2002
EXTENSIONS
More terms from R. J. Mathar, Nov 12 2006
STATUS
approved