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A247122 Primes p such that the digit sum of p is an odd composite number. 1
997, 1699, 1789, 1879, 1987, 2689, 2797, 2887, 3499, 3697, 3769, 3877, 3967, 4597, 4759, 4957, 4993, 5479, 5569, 5659, 5749, 5839, 5857, 6199, 6379, 6397, 6469, 6577, 6793, 6829, 6883, 6991, 7297, 7369, 7459, 7477, 7549, 7639, 7873, 7927, 7963, 8089, 8179, 8269, 8287, 8377, 8467, 8539, 8629, 8647, 8719, 8737, 8863, 8971, 8999 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Motivation from James Grime: "What is the smallest prime with digital sum odd, but not prime?"

This sequences differs from A106763 at a(55) = 8999.

The digit sums are multiples of primes > 3. If the digit sum is a multiple of 3, the number itself cannot be prime.

The first odd composite digit sums are 25 (first occurrence is for 997), 35 (first occurrence is for 8999), 49 (first occurrence is for 598999), 55 (first occurrence is for 2998999), 65 (first occurrence is for 29999999), 77 (first occurrence is for 699899999) ...

LINKS

Michael De Vlieger, Table of n, a(n) for n = 1..10000

EXAMPLE

997 is prime but its digit sum is 25, which is odd and composite. So 997 is a member of this sequence.

MATHEMATICA

a247122[n_Integer] := Flatten@Last@Reap[Module[{i, digitSum}, digitSum[x_] := Plus @@ IntegerDigits[x]; For[i = 1, i < n,

If[OddQ[digitSum[Prime[i]]] && CompositeQ[digitSum[Prime[i]]],

Sow[Prime[i]], Null], i++]]]; a247122[1200] (* Michael De Vlieger, Nov 29 2014 *)

dsocQ[n_]:=Module[{s=Total[IntegerDigits[n]]}, OddQ[s]&&CompositeQ[s]]; Select[Prime[Range[1200]], dsocQ] (* Harvey P. Dale, Feb 21 2016 *)

PROG

(PARI) forprime(p=1, 10^4, if(!isprime(sumdigits(p))&&sumdigits(p)%2, print1(p, ", ")))

CROSSREFS

Cf. A106763, A007953.

Sequence in context: A234847 A057698 A106763 * A091362 A244546 A091365

Adjacent sequences:  A247119 A247120 A247121 * A247123 A247124 A247125

KEYWORD

nonn,base,easy

AUTHOR

Derek Orr, Nov 19 2014

STATUS

approved

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Last modified November 13 17:34 EST 2019. Contains 329106 sequences. (Running on oeis4.)