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A272069 Odd numbers n such that 3^n+1 is a sum of two squares. 0
1, 5, 13, 65, 149, 281, 409, 421, 449, 461, 577, 761 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Keenan Curtis found the values up though 577 in his undergraduate thesis (working with Jeremy Rouse at Wake Forest University). Keenan proved that if 3^n+1 is a sum of two squares for n odd, then n must be equivalent to 1 mod 4, that n itself is a sum of two squares, and that 3^p+1 is a sum of two squares for all primes p dividing n.

REFERENCES

Keenan Curtis, "Sums of Two Squares: An Analysis of Numbers of the form 2^n+1 and 3^n+1", submitted to INVOLVE.

LINKS

Table of n, a(n) for n=1..12.

Greg Dresden, Kylie Hess, Saimon Islam, Jeremy Rouse, Aaron Schmitt, Emily Stamm, Terrin Warren, Pan Yue, When is a^n+1 the sum of two squares?, arXiv:1609.04391 [math.NT], 2016. See p. 20.

S. S. Wagstaff, Jr., The Cunningham Project

EXAMPLE

3^1+1 = 4 = 0^2 + 2^2, so 1 is a term;

3^5+1 = 244 = 10^2 + 12^2, so 5 is a term;

3^13+1 = 1594324 = 82^2 + 1260^2, so 13 is a term.

CROSSREFS

Intersection of A000404 and A034472.

Sequence in context: A149574 A301634 A309167 * A018678 A149575 A156101

Adjacent sequences:  A272066 A272067 A272068 * A272070 A272071 A272072

KEYWORD

hard,more,nonn

AUTHOR

Greg Dresden, Apr 19 2016

EXTENSIONS

a(12) = 761 added from the Cunningham Project via Greg Dresden, Jul 23 2016

STATUS

approved

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Last modified January 27 23:50 EST 2020. Contains 331301 sequences. (Running on oeis4.)