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A232537 Primes p of the form penta(n)-3, where penta(n) is the n-th pentagonal number. 1
2, 19, 67, 89, 173, 373, 587, 1423, 2377, 2749, 2879, 4027, 4507, 4673, 5189, 6899, 7523, 8623, 9319, 10289, 12373, 12647, 13487, 14947, 15859, 17117, 18757, 19777, 20123, 21179, 24509, 25673, 27673, 28909, 29327, 32779, 34123, 38317, 39769, 47969, 52919, 54623 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

The n-th pentagonal number is (3*n^2-n)/2 = n*(3*n-1)/2.

LINKS

K. D. Bajpai, Table of n, a(n) for n = 1..8500

EXAMPLE

a(2)= 19: n= 4: (3*n^2-n)/2-3= 19, which is prime.

a(6)= 373: n= 16: (3*n^2-n)/2-3= 373, which is prime.

MAPLE

KD:= proc() local a, b; a:= (3*n^2-n)/2;  b:=a-3; if isprime(b) then RETURN (b):  fi;  end:  seq(KD(), n=1..500);

MATHEMATICA

Select[Table[(n(3n-1))/2-3, {n, 2, 200}], PrimeQ] (* Harvey P. Dale, Jul 11 2015 *)

CROSSREFS

Cf. A000326 (pentagonal numbers), A000040 (primes).

Sequence in context: A204219 A042149 A218547 * A309341 A079773 A217082

Adjacent sequences:  A232534 A232535 A232536 * A232538 A232539 A232540

KEYWORD

nonn

AUTHOR

K. D. Bajpai, Nov 25 2013

STATUS

approved

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Last modified January 26 01:48 EST 2020. Contains 331270 sequences. (Running on oeis4.)