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1 together with the Pythagorean primes.
1

%I #21 Sep 08 2022 08:46:18

%S 1,5,13,17,29,37,41,53,61,73,89,97,101,109,113,137,149,157,173,181,

%T 193,197,229,233,241,257,269,277,281,293,313,317,337,349,353,373,389,

%U 397,401,409,421,433,449,457,461,509,521,541,557,569,577,593,601,613,617

%N 1 together with the Pythagorean primes.

%C Positive noncomposite numbers of the form 4k + 1.

%C Positive noncomposite numbers in A020668.

%C Essentially the same as A002313 and A002144. - _R. J. Mathar_, Jan 04 2017

%F A002144 UNION {1}. - _R. J. Mathar_, Jan 04 2017

%o (Magma) [1] cat [a: n in [0..200] | IsPrime(a) where a is 4*n + 1 ];

%o (PARI) is(n)=if(isprime(n), n%4==1, n==1) \\ _Charles R Greathouse IV_, Oct 10 2018

%Y Cf. A002144, A006005, A008578, A020668.

%K nonn,easy,less

%O 1,2

%A _Juri-Stepan Gerasimov_, Dec 25 2016