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A002973 a(n) is half of the even member of {x,y}, where x^2 + y^2 is the n-th prime of the form 4i+1.
(Formerly M0135)
15
1, 1, 2, 1, 3, 2, 1, 3, 4, 4, 2, 5, 5, 4, 2, 5, 3, 1, 5, 6, 7, 1, 4, 2, 8, 5, 7, 8, 1, 6, 7, 8, 9, 4, 9, 5, 3, 10, 10, 7, 6, 10, 2, 5, 11, 10, 5, 7, 10, 12, 4, 12, 9, 8, 2, 11, 3, 6, 13, 13, 11, 1, 13, 10, 6, 11, 13, 14, 7, 5, 9, 2, 3, 8, 10, 12, 5, 14, 2, 3, 14, 11, 15, 16, 16, 5, 15, 1, 8, 11 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

a(n) is odd iff x^2 + y^2 == 5 (mod 8). [Vladimir Shevelev, Jul 12 2009]

A002972(n)^2 + 4*a(n)^2 = A002144(n); A002331(n+1) = Min(A002972(n),2*a(n)) and A002330(n+1) = Max(A002972(n),2*a(n)). [Reinhard Zumkeller, Feb 16 2010]

REFERENCES

E. Kogbetliantz and A. Krikorian, Handbook of First Complex Prime Numbers, Gordon and Breach, NY, 1971, p. 243.

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

LINKS

Rainer Rosenthal, Table of n, a(n) for n = 1..10000, first 1000 terms from T. D. Noe.

S. R. Finch, Powers of Euler's q-Series, arXiv:math/0701251 [math.NT], 2007.

E. Kogbetliantz and A. Krikorian, Handbook of First Complex Prime Numbers, Gordon and Breach, NY, 1971. [Annotated scans of a few pages]

FORMULA

a(n) = Min(A173331(n), A002144(n) - A173331(n)) / 2. [Reinhard Zumkeller, Feb 16 2010]

EXAMPLE

The 3rd prime of the form 4i+1 is 17 = 1^2 + 4^2, so a(3) = 4/2 = 2.

MATHEMATICA

pmax = 1000; k[p_] := Module[{k, m}, k /. ToRules[Reduce[k>0 && m >= 0 && (2k)^2 + (2m+1)^2 == p, {k, m}, Integers]]]; For[n=1; p=5, p<pmax, p = NextPrime[p], If[Mod[p, 4]==1, a[n] = k[p]; Print["a(", n, ") = ", a[n]]; n++]]; Array[a, n-1] (* Jean-François Alcover, Feb 26 2016 *)

PROG

(PARI) \\ use function decomp2sq from A002972

forprime (p=5, 1000, if (p%4==1, print1(select(x->!(x%2), decomp2sq(p))[1]/2, ", "))) \\ Hugo Pfoertner, Aug 27 2022

CROSSREFS

Cf. A002144, A002972, A005098.

Sequence in context: A124458 A199538 A324337 * A071476 A071499 A039953

Adjacent sequences: A002970 A002971 A002972 * A002974 A002975 A002976

KEYWORD

nonn

AUTHOR

N. J. A. Sloane

EXTENSIONS

Better description from Jud McCranie, Mar 05 2003

STATUS

approved

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Last modified February 6 16:56 EST 2023. Contains 360110 sequences. (Running on oeis4.)